Illuminating Gravitational Waves:A Concordant Picture of Photons

from a Neutron Star Merger

M. M. Kalila1∗, E. Nakar2, L. P. Singer3,4, D. L. Kaplan5,D. O. Cook1, A. Van Sistine5, R. M. Lau1, C. Fremling1,O. Gottlieb2, J. E. Jencson1, S.M. Adams1, U. Feindt6, K.

Hotokezaka7, S. Ghosh5, D. A. Perley8, P.-C. Yu9, T. Piran10,J. R. Allison11,12, G. C. Anupama13, A.Balasubramanian14,K. W Bannister15, J. Bally16, J. Barnes17, S. Barway18, E.

Bellm19, V. Bhalerao20, D. Bhattacharya21, N. Blagorodnova1,J. S. Bloom22,23, P. R. Brady5, C. Cannella1, D. Chatterjee5,S. B. Cenko3,4, B. E. Cobb24, C. Copperwheat8, A. Corsi25,K. De1, D. Dobie11,26,15, S. W. K. Emery27, P. A. Evans28,O. D. Fox29, D. A. Frail30, C. Frohmaier31,32, A. Goobar6,

G. Hallinan1, F. Harrison1, G. Helou33, T. Hinderer34,A. Y. Q. Ho1, A. Horesh4, W.-H. Ip9, R. Itoh35, D. Kasen22,36,

H. Kim37, N.P.M. Kuin27, T. Kupfer1, C. Lynch11,26,K. Madsen1, P. A. Mazzali8,38, A. A. Miller39,40, K. Mooley41,T. Murphy11,26, C.-C. Ngeow9, D. Nichols34, S. Nissanke34,

P. Nugent22,23, E. O. Ofek42, H. Qi5, R. M. Quimby43,44,S. Rosswog45, F. Rusu46, E. M. Sadler11,26, P. Schmidt34,J. Sollerman45, I. Steele8, A. R. Williamson34, Y. Xu1, L.

Yan1,33, Y. Yatsu35, C. Zhang5, W. Zhao46

1Division of Physics, Math and Astronomy, California Institute of Technology,1200 East California Boulevard, Pasadena, CA 91125, USA2The Raymond and Beverly Sackler School of Physics and Astronomy,Tel Aviv University, Tel Aviv 69978, Israel3Astroparticle Physics Laboratory, NASA Goddard Space Flight Center,

1

Mail Code 661, Greenbelt, MD 20771, USA4Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA∗To whom correspondence should be addressed; E-mail: mansi@astro.caltech.edu. 5Departmentof Physics, University of Wisconsin, Milwaukee, WI 53201, USA6The Oskar Klein Centre, Department of Physics, Stockholm University,AlbaNova, SE-106 91 Stockholm, Sweden7Center for Computational Astrophysics, Simons Foundation,162 5th Ave, New York, 10010, NY, USA8Astrophysics Research Institute, Liverpool John Moores University,IC2, Liverpool Science Park, 146 Browlow Hill, Liverpool, L3 5RF, UK9Graduate Institute of Astronomy, National Central University,No. 300, Zhongda Rd., Zhongli Dist., Taoyuan City 32001, Taiwan10Racah Institute of Physics, The Hebrew University of Jerusalem,Jerusalem, 91904, Israel11Sydney Institute for Astronomy, School of Physics A28,The University of Sydney, NSW 2006, Australia12ARC Centre of Excellence for All-sky Astrophysics in 3 Dimensions (ASTRO 3D)13Indian Institute of Astrophysics, II Block Koramangala, Bangalore-560034, India14Indian Institute of Science Education and Research,Dr. Homi Bhabha Road, Pashan, Pune 411008, India15Australia Telescope National Facility, Commonwealth Scientific and IndustrialResearch Organisation, Astronomy and Space Science,PO Box 76, Epping, NSW 1710, Australia16University of Colorado, Boulder, USA17Columbia Astrophysics Laboratory, Columbia University,New York, NY, 10027, USA18South African Astronomical Observatory (SAAO),P.O. Box 9, Observatory, Cape Town 7935, South Africa19Department of Astronomy, University of Washington, Seattle, WA 9819520Department of Physics, Indian Institute of Technology Bombay,Mumbai 400076, India21Inter-University Centre for Astronomy and Astrophysics,P. O. Bag 4, Ganeskhind, Pune 411007, India22Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA23Lawrence Berkeley National Laboratory, 1 Cyclotron Road,MS 50B-4206, Berkeley, CA 94720, USA24Department of Physics, George Washington University,Washington, DC 20052, USA25Department of Physics and Astronomy, Texas Tech University,Box 41051, Lubbock, TX 79409-1051, USA26Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO),

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27University College London, Mullard Space Science Laboratory,Holmbury St. Mary, Dorking, RH5 6NT, U.K.28University of Leicester, X-ray and Observational Astronomy Research GroupLeicester Institute for Space and Earth Observation, Department of Physics& Astronomy,University Road, Leicester, LE1 7RH, UK 29Space Telescope Science Institute, 3700 San Mar-tin Dr, Baltimore, MD 2121830National Radio Astronomy Observatory, Socorro, New Mexico, USA31Department of Physics and Astronomy, University of Southampton,Southampton, Hampshire SO17 1BJ, UK32Institute of Cosmology and Gravitation, Dennis Sciama Building,University of Portsmouth, Burnaby Road, Portsmouth PO1 3FX, UK33Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA91125, USA34Institute of Mathematics, Astrophysics and Particle Physics,Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands35Department of Physics, Tokyo Institute of Technology,2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan36Department of Physics, University of California, Berkeley,CA 94720, USA37Gemini Observatory, Casilla 603, La Serena, Chile38Max-Planck Institute for Astrophysics, Garching, Germany39Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA)and Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA40The Adler Planetarium, Chicago, IL 60605, USA41Astrophysics, Department of Physics, University of Oxford,Keble Road, Oxford OX1 3RH, UK42Department of Particle Physics & Astrophysics, Weizmann Institute of Science,Rehovot 7610001, Israel43Department of Astronomy, San Diego State University, CA92182, USA44Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University ofTokyo Institutes for Advanced Study, The University of Tokyo,Kashiwa, Chiba 277-8583, Japan45 The Oskar Klein Centre, Department of Astronomy, Stockholm University,AlbaNova, SE-106 91 Stockholm, Sweden46University of California Merced, USA∗To whom correspondence should be addressed; E-mail: mansi@astro.caltech.edu.

3

Merging neutron stars offer an exquisite laboratory for simultaneously study-

ing strong-field gravity and matter in extreme environments. We establish the

physical association of an electromagnetic counterpart (EM170817) to gravita-

tional waves (GW170817) detected from merging neutron stars. By synthesiz-

ing a panchromatic dataset, we demonstrate that merging neutron stars are a

long-sought production site forging heavy elements by r-process nucleosynthe-

sis. The weak gamma-rays seen in EM170817 are dissimilar to classical short

gamma-ray bursts with ultra-relativistic jets. Instead, we suggest that break-

out of a wide-angle, mildly-relativistic cocoon engulfing the jet elegantly ex-

plains the low-luminosity gamma-rays, the high-luminosity ultraviolet-optical-

infrared and the delayed radio/X-ray emission. We posit that all merging neu-

tron stars may lead to a wide-angle cocoon breakout; sometimes accompanied

by a successful jet and sometimes a choked jet.

On 2017 August 17 at 12:41:04 UTC, gravitational waves from the merger of two neu-

tron stars (NS-NS) were detected by the Laser Interferometer Gravitational-Wave Observatory

(LIGO) and dubbed GW170817 (1). Two seconds later, the first temporally-coincident photons

were detected as γ-rays by the Fermi satellite (2–4). GW170817 was such a loud event that the

joint on-sky localization from the LIGO and Virgo interferometers was only 31 square degrees

(Figure 1) with an initial distance estimate of 40±8 Megaparsec (5). To identify potential host

galaxies (6), we cross-matched to our Census of the Local Universe (CLU; (7)) galaxy catalog

and found only 49 galaxies in this volume (8). To prioritize follow-up, we ranked the galaxies

by stellar mass (see Table S1 and supplementary online text (8)). A multitude of telescopes

promptly began multi-wavelength searches for an electromagnetic counterpart in and around

these galaxies. Half a day later, a bright optical transient was identified and announced by the

Swope telescope team at Las Campanas Observatory (9, 10) in the 3rd ranked galaxy in our

4

list, named NGC 4993. This source, SSS 17a, is located at right ascension 13h09m48.071s and

declination −23d22m53.37s (J2000 equinox, (8)), with a projected offset from the nucleus of

NGC 4993 of 2.2 kiloparsec and away from any sites of star formation (Figure S1; (8)). We also

detect this transient in the infrared and ultraviolet wavelengths (see companion paper; (11)).

Nine days later, an X-ray counterpart was identified (12). Fifteen days later, a radio counterpart

was identified (see companion paper; (13)).

Initially, the bright luminosity and the blue, featureless optical spectrum of SSS17a ap-

peared to be consistent with a young supernova explosion that should brighten (see Figure S2,

Figure S3). However, on the second night, the source faded substantially in the optical and

brightened in the infrared (Figure 4). Combining ultraviolet-optical-infrared (UVOIR) data

from 24 telescopes on 7 continents, we constructed a bolometric light curve (Figure 2; see (8)

for details). The bolometric luminosity evolves from 1042 erg s−1 at 0.5 d to 3×1040 erg s−1

at 10 d (Figure 2). By estimating the black-body effective temperature evolution, we find that

the source rapidly cools from ≈ 11000 K to ≈ 5000 K in a day to ≈1400 K in ten days. The

inferred photospheric expansion velocities span 0.3c to 0.1c, where c is the speed of light (8).

Furthermore, infrared spectroscopy shows broad features that are unlike any other transient seen

before (Figure 3, Figure S4, Figure S5). The combination of high velocities, fast optical decline,

slow infrared evolution and broad peaks in the infrared spectra are unlike any other previously

known transient and unlikely to be due to a chance coincidence of an unrelated source. We thus

establish that the panchromatic photons, hereafter EM170817, are spatially, temporally and

physically associated with GW170817. With this firm connection, we now turn our attention to

understanding the astrophysical origin of EM170817.

5

Evidence for Nucleosynthesis of Heavy Elements

It is well established that chemical elements up to iron in the periodic table are produced either

in the Big Bang or in cores of stars or in supernova explosions. However, the origin of half

the elements heavier than iron, including gold, platinum and uranium, has remained a mystery.

These heavy elements are synthesized by the rapid capture of neutrons (r-process nucleosyn-

thesis). Some models have proposed that the decompression of neutron-rich matter in a NS-NS

merger may provide suitable conditions to robustly synthesize heavy r-process elements (14,15).

Radioactive decay of freshly synthesized unstable isotopes should drive transient electromag-

netic emission known as a “kilonova” or “macronova” (e.g, (16, 17)). We test this hypothesis

with the optical and infrared data of EM170817.

First, we compare the spectra of EM170817 to a library of astronomical transients (8) and

theoretical models for macronova spectra (18). The optical spectra exhibit a featureless con-

tinuum (Figure S2, Figure S3). Infrared spectra (Figure 3) have two distinct, broad peaks in

J-band (10620 ± 1900 Angstrom) and H-band (15500 ± 1430 Angstrom). Due to the high

velocities in the ejecta material, each peak may be produced by a complex blend of elements

instead of a single element. Although the J-band peak is reminiscent of either Helium or Hy-

drogen, the corresponding feature in theH-band seen in core-collapse supernovae is not present

(Figure S5). If instead we compare to Type Ia supernovae, the J-band peak could be similar to

iron group elements. However, once again the second H-band peak is dissimilar to that seen

in Type Ia supernovae (Figure S4). By comparing predictions of spectra of macronovae (18),

based on the assumption that Neodymium (Nd) is representative of lanthanides synthesized via

the r-process, we find a reasonable match to both the J-band and H-band features for ejecta

mass (Mej) of 0.05 M� and velocity (v) of 0.1c (Figure 3). Recent updates to these models,

incorporating line transitions from 14 elements and tuning the relative abundance ratios, indi-

6

cate that Nd plays a crucial role in explaining these features (19). We conclude that a blend of

elements substantially heavier than elements produced in supernovae is a viable explanation for

the spectra of EM170817.

Next, we compare our infrared light curves of EM170817 (Figure 4) to a suite of existing

macronova models by various groups (20–23). The slow, red photometric evolution seen in

EM170817 is a generic feature of all macronova models despite their differing treatments of

matter dynamics, matter geometry, nuclear heating, opacities and radiation transfer. The ob-

served late-time emission (>3 d) is fully consistent with radioactive decay of the dynamical

ejecta containing elements from all three r-process abundance peaks (Figure S10). The ob-

served luminosity, temperature and temporal evolution roughly matches model predictions for

an ejecta mass of ∼ 0.05 solar masses (M�), an ejecta velocity of ∼ 0.1c and an opacity (κ) of

∼ 10 cm2 g−1.

We examine this match further with simple analytics. Dividing the observed bolometric

luminosity (≈ 6 × 1041 erg s−1 at 1 day) by the beta-decay heating rate of r-process elements

(≈ 1.5 × 1010 erg s−1 g−1; (24)) gives a lower limit on the r-process ejecta mass of >0.02

M�. The decline rate of the bolometric luminosity also matches that expected from the beta-

decay heating rate of r-process elements with the time-dependent thermalization efficiency of

the decay products (Figure 2). The expansion velocity of the ejecta, 0.1–0.3c, derived from the

photospheric radius is consistent with the results of merger simulations (25–27). Ejecta mass

estimates based on observed emission are necessarily lower limits as a significant amount of

additional matter can be hidden at lower velocity.

Next, we focus on the early-time emission of EM170817 that is hotter, more luminous

and faster-rising than predicted by the suite of macronova model predictions discussed above

(Figure 4). Decay of free neutrons would give an unphysically large ratio of neutron mass to

ejecta mass (8). Ultraviolet flashes predicted by (28, 29) are on much shorter timescale than

7

observed for EM170817. Instead, we propose two possible explanations: (i) If some fraction of

the ejecta is boosted to mildly relativistic speeds, the relativistic expansion shortens the observed

peak time and the Doppler effect results in bluer, brighter emission. The jet cocoon model

(see below) can accelerate enough material at higher latitudes. All material would have κ>

≈ 1 cm2 g−1 in this scenario. (ii) A disk-driven wind enriched with lighter r-process elements

with κ ≈ 0.5 cm2 g−1 could also produce early, blue emission (11). This wind could be driven

from a merger remnant that is a massive neutron star with an accretion torus. We could have

distinguished between these two possibilities if data were available at even earlier times.

A Synthesized Model Explaining the Panchromatic Photons

We discuss three models in an effort to build a self-consistent picture that explains the γ-ray,

X-ray, ultraviolet, optical, infrared and radio photons (Figure 5).

A Classical, On-Axis Short Hard Gamma Ray Burst: Ruled Out

A classical short hard gamma ray burst (sGRB) is produced by a jet in the line-of-sight of the

observer (Model A in Figure 5) that is narrow (opening angle θjet ∼ 10◦) and ultra-relativistic

(Lorentz factor Γ >∼ 100). The progenitors of sGRBs have long been hypothesized to be NS-

NS mergers (30). However, the observed γ-ray luminosity of EM170817 (∼1047 erg s−1, (3,4))

is lower than typical sGRBs by four orders of magnitude (31, 32). If EM170817 were simply

an extremely weak sGRB, then the successful breakout of a narrow, ultra-relativistic jet would

require<3×10−6 M� of material that was previously ejected in the direction of the jet (8). If the

jet opening angle were wider, it would require even less material to successfully break out (8).

Such a low ejecta mass is in contradiction with the observed bright UVOIR counterpart, which

indicates ≈ 0.05 M� of ejecta. Furthermore, this scenario cannot account for the delayed onset

of X-ray emission (11) and radio emission (13).

8

A Classical, Off-Axis Short Hard Gamma Ray Burst: Unlikely

Next, we consider the possibility of a classical off-axis sGRB where the observer is not in the

line-of-sight of a strong, ultra-relativistic jet (Model B in Figure 5). Given the sharp drop in

observed γ-ray luminosity with observing angle, we find that the observer could only be off-

axis by<8◦ (8). Such a slightly off-axis orientation is unlikely as only a small fraction (≈5%) of

observing angles are consistent with the observational constraints. Moreover, in this scenario,

EM170817 is expected to exhibit a bright afterglow at all wavelengths roughly one day after

the NS-NS merger, when the external shock decelerates to Γ ∼ 10. Initial non-detections in the

radio (13) and X-ray (11) observations at this phase constrain the circummerger environment

to an implausibly low density (<10−6 cm−3). Another problem is that a hypothetical on-axis

observer to such a sGRB would expect to see photons harder than we have thus far seen in

sGRBs (8). Thus, it is unlikely that the γ-rays are produced by a slightly off-axis sGRB.

We conclude that EM170817 is not similar to the classical population of previously observed

sGRBs. While the observed γ-rays are indicative of a relativistic outflow (with or without a

jet), they must originate in a different physical mechanism (8). We explore the possibility of a

structured jet in sGRBs with a distribution of Lorentz factors and identify multiple challenges

with this model (8). Therefore, next, we propose a model with a wide-angle mildly relativistic

outflow that propagates in our direction with a relatively small Lorentz factor.

Cocoon Breakout: A concordant picture

Based on our UVOIR observations, we estimate that a few hundredths of a solar mass of ejecta

are propelled into the circummerger medium of a NS-NS merger with velocities spanning a

few tenths the speed of light. We consider a model where a relativistic jet is launched after a

short delay, perhaps on account of a delayed collapse of the hyper-massive neutron star into

a black hole. As the jet drills through the ejecta, the material enveloping the jet inflates to

9

form a pressurized cocoon that expands outward at mildly relativistic speeds. There are two

possibilities: If the jet is wide-angle (≈30◦), it will become choked and fail to drill out (Model

C in Figure 5). If the jet is narrow (≈10◦) and long-lived, it could penetrate the ejecta and look

like a classical sGRB to an on-axis observer (Model D in Figure 5).

Independent of the fate of the jet that created the cocoon, recent numerical simulations

(29) show that the cocoon would expand at mildly relativistic velocities (Γ ≈ 2 − 3 ) over

a wide opening angle (≈ 40o) with energy comparable to the jet. The cocoon has a wide

enough angle and sufficient kinetic energy to easily explain the observed γ-rays. However,

it remains unclear how a cocoon would dissipate its energy internally at the radius where γ-

rays are observed, given its ballistic and homologous expansion (unlike sGRB jets which are

expected to be variable with irregular internal velocities and structure that can dissipate the jet

energy by internal shocks or magnetic reconnection). A wide angle mildly relativistic cocoon,

found by (29), was recently proposed as a source of wide-angle γ-ray emission (33). However,

this was based on an ad-hoc dissipation process that is somehow at work near the photosphere

(33). Here, we suggest that the dissipation mechanism is the interaction of the cocoon with

the ejecta and that the observed γ-rays result from the breakout of the mildly relativistic shock

(driven by the cocoon) from the leading edge of the ejecta. We find that such a breakout can

explain all properties of the observed low-luminosity γ-rays if its Lorentz factor is ≈ 2− 3 and

the breakout radius is ∼ 3× 1011cm (8).

We performed a relativistic hydrodynamical simulation in which a jet is injected into ex-

panding ejecta to verify this picture for EM170817 (8). We find that even if a minute amount

of ejecta (≈ 3× 10−9M�) moves at 0.8c, the breakout radius and velocity match those needed

to produce the observed γ-rays for a wide range of ejecta and jet properties (8). For example,

in the simulation shown in Figure 6, a shock with Γ ≈ 2.5 breaks out 10 s after the merger at a

radius of 2.4 × 1011cm, generating γ-ray emission that would be observed with a delay of 2 s

10

with respect to merger time (consistent with the Fermi observations; (3, 4)). After the cocoon

breaks out, the photons that were deposited by the shock diffuse outwards and produce cooling

emission that fades on timescales of hours (29). After a few hours, radioactive decay of r-

process elements becomes the dominant source of the observed emission. The emission during

the first day is dominated by fast cocoon material (v≈0.4c), which is composed of high-latitude,

low-opacity (κ ∼1 cm2 g−1) ejecta that was accelerated by the jet to high velocities. After a

few days, the slower, higher-opacity (κ ∼10 cm2 g−1) dynamical ejecta begins to dominate

the emission. We find that the bolometric light curve evolution and the temperature evolution

predicted by this simulation is consistent with our UVOIR observations (Figure 2).

The available radio and X-ray data are broadly consistent with both cocoon scenarios albeit

with slightly different circummerger densities (11, 13). If the jet is choked, the radio and X-

ray data could be explained by the forward shock that the expanding cocoon drives into the

circummerger medium. If the jet is successful, the radio and X-ray data could be explained as

a widely off-axis afterglow of the jet. If this emission is from the forward shock of a cocoon,

we predict that the X-rays and radio will continue to rise. On the other hand, if this emission is

from a widely off-axis afterglow of the jet, we predict that it will evolve slowly and eventually

fade. In both scenarios, a cocoon would be needed to explain the γ-rays. We conclude that

the cocoon model can self-consistently explain the multi-wavelength properties of EM170817

spanning γ-rays to radio.

A “Bright” Future

Now we consider the question of whether EM170817 was an exceptional event or whether

multi-messenger detections will soon become routine. The large ejecta masses and high ve-

locities seen in EM170817 suggests that intrinsically luminous UVOIR macronova emission

should accompany every NS-NS merger. If our proposed mildly relativistic cocoon model is

11

correct, the wide opening angle of the cocoon implies that γ-rays would be emitted towards

the observer in about 30% of NS-NS mergers. If the jet is choked, we expect to see late onset

of radio and X-ray emission from the cocoon forward shock. If the jet producing the cocoon

successfully breaks out, the source would appear either as a classical wide off-axis afterglow

or a classical on-axis afterglow depending on the observer’s line-of-sight. The launch of a suc-

cessful on-axis cocoon jet may already have been seen in previous reports of possible late-time

excess optical/infrared emission in sGRBs attributed to macronovae. In Figure 4, we find that

the excess seen in GRB 130603B (34), GRB 160821B (35) and GRB 050709 (36) are roughly

consistent with our observed light curve for EM170817. Separately, a plateau in the distribution

of durations of sGRB may indicate that a large fraction of sGRBs may have choked jets (37).

Joint gravitational wave and electromagnetic observations of NS-NS mergers will shed light on

the relative fraction of cocoons with choked jets and cocoons with successful jets.

Now we consider whether NS-NS mergers could be the primary sites of r-process nu-

cleosynthesis. This depends on both the rate of NS-NS mergers and the average amount

of r-process material synthesized per merger. Based on the macronova light curve, we es-

timated a lower limit on the mass of the produced r-process elements in EM170817 to be

Mej ≈ 0.05M�. The solar abundance pattern shows that the first of three r-process peaks

accounts for ≈80% of the total r-process abundance (Figure S10 and (8)). To account for the

observed solar abundance in all three r-process peaks with NS-NS mergers, we would need a

rate of ∼ 500 Gpc−3 yr−1 (Mej/0.05M�)−1. To account for the observed abundance in the two

heavier r-process peaks with NS-NS mergers, the rate would only need to be∼ 100 Gpc−3 yr−1.

Based on the detection of GW170817, a NS-NS merger rate of 320–4740 Gpc−3 yr−1 was

estimated at 90% confidence (1). This is larger than the classical sGRB beaming-corrected

rate (38,39) and larger than the predicted fraction of NS-NS mergers based on the Galactic pop-

ulation (40). Based on an archival search for transients like EM170817 in the Palomar Transient

12

Factory database, we find a 3-σ upper limit on the rate of 800 Gpc−3 yr−1 (8). Therefore the

large ejecta mass of EM170817 and the high rate estimates of GW170817/EM170817 are con-

sistent with the scenario that NS-NS mergers are the main production sites of r-process elements

of the Milky Way (as predicted by (14)).

The large rate, the wide angle for contemporaneous γ-rays, the bright UVOIR emission,

the forward shock giving a late onset of X-rays and radio, the increase in sensitivity of GW

interferometers, the increase in sensitivity of EM facilities (e.g. (41–44)) — all bode well for a

“bright” photon-laden EM+GW future.

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Acknowledgments

We thank Iva Kostadinova for seamlessly coordinating the GROWTH (Global Relay of Obser-

vatories Watching Transients Happen) program and Britt Griswold for beautiful graphic arts.

We thank Patricia Whitelock for facilitating IRSF observations. We thank Scott Barthelmy

for setting up an LVC GCN system that facilitated quick, citable communication between as-

tronomers and maximized the science return. We thank Sterl Phinney, Shri Kulkarni and Lars

Bildsten for valuable comments. We thank the staff of Gemini Observatory, in particular the

director Laura Ferrarese for rapidly approving our Director’s Discretionary Time request, and

our program contact scientists Mischa Shirmer, Hwihyun Kim, Karleyne Silva, Morten Ander-

son, and Ricardo Salinas for supporting and executing observations. We especially grateful to

Gemini for postponing scheduled maintenance on the FLAMINGOS-2 instrument in order to

obtain as much data as possible on this extraordinary event.

This work was supported by the GROWTH (Global Relay of Observatories Watching Tran-

sients Happen) project funded by the National Science Foundation under PIRE Grant No 1545949.

24

GROWTH is a collaborative project among California Institute of Technology (USA), Univer-

sity of Maryland College Park (USA), University of Wisconsin Milwaukee (USA), Texas Tech

University (USA), San Diego State University (USA), Los Alamos National Laboratory (USA),

Tokyo Institute of Technology (Japan), National Central University (Taiwan), Indian Institute

of Astrophysics (India), Inter-University Center for Astronomy and Astrophysics (India), Weiz-

mann Institute of Science (Israel), The Oskar Klein Centre at Stockholm University (Sweden),

Humboldt University (Germany), Liverpool John Moores University (UK). The data presented

here is available in observatory archives and the PLUTO simulation input and output files are

available online (URLs in supplementary online text). Additional acknowledgements are in the

supplementary online text.

Supplementary materials

www.sciencemag.org

Materials and Methods

Supplementary Text

Figs. S1, S2, S3, S4, S5, S6, S7, S8, S9, S10

Table S1, S2, S3

References (48-175)

Movie S1

25

A B

D

C

IPN

HL

HLV

GBM

90%

90%

50%

50%

Right ascension

Right ascension

Dec

linat

ion

Dec

linat

ion

x´x´

x

x10Mpc

5°

10´

60°

30°

0°

-30°

-60°

16h 12h8h

Gemini+F2 JHKs

Lum

inosit

y dist

ance

Figure 1: Localization of GW170817 and associated transient EM170817. (A) Constraints atthe 90% confidence level on the sky position from gravitational-wave and γ-ray observations.The rapid LIGO localization is indicated by the green dashed contour, and the LIGO/Virgolocalization by solid green. Fermi GBM (4) is shown in orange, and the Interplanetary Networktriangulation from Fermi and INTEGRAL in blue (45). The shaded region is the Earth limbas seen by AstroSat which is excluded by the non-detection by the Cadmium Zinc TellurideImager instrument. (B) 49 galaxies from the Census of the Local Universe catalog (Table S3;red, with marker size proportional to the stellar mass of the galaxy) within the LIGO/Virgothree-dimensional 50% and 90% credible volumes (green). One radio-selected optically-darkgalaxy whose stellar mass is unknown is marked with a +. (C) Cross-section along the X-X’ plane from panel B, showing the luminosity distances of the galaxies in comparison to theLIGO/Virgo localization. (D) False-color near-infrared image of EM170817 and its host galaxyNGC 4993, assembled from near-infrared observations with the FLAMINGOS-2 instrument onGemini-South (8), with J , H , and Ks shown as blue, green, and red, respectively. Our Ks-banddetections span 2017 Aug 18.06 to 2017 Sep 5.99 and we show 2017-08-27.97 above.

26

L (e

rg s−1

)

A

12

3

1041

1042

13579

1113

T (1

03 K)

B

02468

R (1

04 R )⊙

C

0 1 2 3 4 5 6 7 8 9 10 11 12 13

0.1

0.2

0.3

Time since GW170817 (days)

v/c

D

Figure 2: The evolution of EM170817 derived from the observed spectral energy distribution.(A) Bolometric luminosity. (B) Blackbody temperature. (C) Photospheric radius. (D) Inferredexpansion velocity. Individual points represent blackbody fits performed at discrete epochs towhich the observed photometry has been interpolated using low-order polynomial fits. Dashedlines represent an independent Markov-Chain Monte Carlo fit without directly interpolatingbetween data points (see (8) for methodology and best-fit parameter values). The solid red lines(in A and B) represent the results of a hydrodynamical simulation of the cocoon model wherethe UVOIR emission is composed of (in A) cocoon cooling (yellow dashed line labeled 1), fastmacronova (>0.4c; green dashed line labeled 2), and slow macronova (<0.4c; blue dashed linelabeled 3).

27

10000 12000 14000 16000 18000 20000 22000 24000Rest wavelength (Å)

10 17

10 16

Flux

den

sity,

f (e

rg s

1 cm

2 Å1 )

Figure 3: Near-infrared spectrum of EM170817 at 4.5 days after merger. For display purposes,the data have been smoothed using a Savitzky-Golay filter (solid black line), and the unfiltereddata are shown in grey. A predicted model macronova spectrum (18) assuming an ejecta massof Mej = 0.05M� and a velocity of v = 0.1c at a phase of 4.5 days post merger is shown inred. The spectra have been corrected for Milky Way extinction assuming reddening E(B −V ) = 0.1 (8). Regions of low signal-to-noise ratio from strong telluric absorption by theEarth’s atmosphere between the near-infrared J , H , and K spectral windows are indicated bythe vertical dark grey bars. The light grey shaded band is the blackbody which best fits thephotometric measurements at 4.5 days (8).

28

Time since GW170817 (days)

15.0

17.5

20.0

22.5

25.0

27.5

30.0

32.5

Appa

rent

mag

nitud

e (A

B)

(A)

UVW1+10UVM2,F225W+7UVW2+6U,F336W,u+7B,g0+6g+5V+4

r0,r,R+3i,i 0,I+2z,z0+1y−0J−1H−2K,Ks−3

0 2 4 6 8 10 12 14 16 18 20

16

18

20

22i mag

. (AB

) GRB050709GRB060614

(D)

16

18

20

22H m

ag. (

AB) GRB130603B

(C)

18

20

22K s m

ag. (

AB) GRB160821B

(B)

−17.5

−15.0

−12.5

−10.0

−7.5

−5.0

−2.5

0.0

Abso

lute

mag

nitud

e (A

B)

−16

−14

−12

−10

Abs. i (

AB)

−16

−14

−12

−10

Abs. H

(AB)

−16

−14

−12

−10

Abs. K s

(AB)

Figure 4: Lightcurves of EM170817. (A) Multi-wavelength lightcurve based on theultraviolet/optical/near-infrared photometry of EM170817 (Table S1 and (8)) plotted as ABmagnitude vs. time since merger, with open triangles indicating 5σ upper limits, colored bywavelength. (B–D) Ks, H , and i-band lightcurves of EM170817 with literature macronovamodel lightcurves, which show a good match in the infrared but fail to produce the observedblue emission. For all lightcurves we plot both apparent magnitude and absolute magnitudeassuming a distance of 40 Mpc. Detections are shown as circles, upper limits as triangles. Themodels have been scaled to a distance of 40 Mpc and reddened with E(B − V ) = 0.1 (8). Themodel lightcurves are the following: Mej = 0.05M�, vej = 0.1c from (46), model N4 with theDZ31 mass formula from (22) and γA2 at a viewing angle of 30◦ from (23). Optical and near-infrared observations of previously observed short GRBs which appeared abnormally brightare shown as squares (scaled to 40 Mpc and corrected for time dilation). GRB080503 (47)would have had to be at a redshift of 0.22 to be consistent. GRB 060614 (48) is too lumi-nous at late times. The excess emission noted in GRB 160821B (35), GRB 130603B (34) andGRB 050709 (36) appear to be similar to EM170817.

29

Inter

sella

r Medium

Inter

sella

r Medium

Inter

sella

r Medium

Inter

sella

r Medium

30°

30° 30°

A On-axis Weak sGRB B Slightly Off-Axis Classical sGRB

C Cocoon with Choked Jet D On-axis Cocoon with Off-Axis Jet

WeaksGRB

sGRB

sGRB

Macronova(UVOIR)

Macronova(UVOIR)

Macronova(UVOIR)

Macronova(UVOIR)

Weak Afterglow(X-ray/Radio)

Afterglow(X-ray/Radio)

Afterglow(X-ray/Radio)

Afterglow(X-ray/Radio)

Weak γ-rays

Weak γ-rays Weak γ-rays

Figure 5: Model schematics considered in this paper. In each panel, the eye indicates the lineof sight to the observer. (A) A classical, on-axis, ultra-relativistic, weak short gamma-ray burst(sGRB). (B) A classical, slightly off-axis, ultra-relativistic, strong sGRB. (C) A wide-angle,mildly-relativistic, strong cocoon with a choked jet. (D) A wide-angle, mildly-relativistic, weakcocoon with a successful off-axis jet.

30

Ejecta

Jet

Cocoon

5×1010 cm3.5 s after merger

Ejecta

Cocoon

5×1010 cm10 s after merger

1014 1016 1018 1020

Energy density, T00 (erg cm 3)=0.2 0.5 0.99

Log four-velocity, log( )

=1.1 2 3 4 5 10

Figure 6: Snapshots from a hydrodynamic simulation of a cocoon generated by a choked jetwith emission consistent with EM170817 (see (8) for details). The left half-plane is color-coded by logarithmic energy density (erg cm−3) and depicts the energetics. The right half-planeis color-coded by logarithmic four-velocity (Γβ) and depicts the kinematics. The observer is atan angle of 40◦, the ejecta mass is 0.1 M� and the jet luminosity is 2.6×1051 erg s−1. Based onthis simulation, a bolometric light curve is calculated and shown in Figure 2. (A) This snapshotis taken at 3.5 s, shortly after the jet injection stops. The jet is fully choked by 4s. (B) Thissnapshot is taken at 10 s when the cocoon breaks out. The breakout radius is 2.4×1011 cmwhich corresponds to 8 light-seconds. Thus, the delay between the observed γ-ray photons andthe NS-NS merger is the difference in these times, 2 s. The Lorentz factor of the shock uponbreakout is between 2 and 3.

31

Materials and Methods

Data Reduction

Below we describe the ultraviolet/optical/near-infrared photometric and spectroscopic observa-

tions, along with the relevant data reduction. The photometric results from our observations

as well as the literature are collected in Table S1. A log of our spectroscopic observations is

provided in Table S2.

32

Gemini-S F2 near-infrared imaging

We observed EM170817 between 2017 August 18 and 2017 September 15 with the FLAMINGOS-

2 imaging spectrograph (49) on the 8.1-m Gemini-S Telescope on Cerro Pachon in Chile (PI

Singer, Programs GS-2017B-DD-1, GS-2017B-DD-6). Near-infrared JHKs imaging data

were reduced using standard procedures and calibrated relative to near-IR sources from the

Two Micron All-Sky Survey (2MASS) (50). Flat fields at each filter position were taken each

night of F2 observations using the Gemini Facility Calibration Unit (GCAL). Dark frames with

identical exposure times to the flat fields and science frames were also taken at least every other

observing night. Median sky images were produced from a series of dithered science frames

and then subtracted to remove the OH and thermal emission from the atmosphere. On nights

with poor observing conditions where the sky varied on timescales faster than a full dither se-

ries, a subset of the science frames were selected to create the sky frame. The sky-subtracted

science frames were then aligned using the centroid of the bright core of the galaxy NGC 4993

and then co-added.

Three 2MASS stars (2MASS J13094767−2321594, 2MASS J13094842−2323499, 2MASS J13094767−2321594)

in the vicinity of EM170817 with the highest photometric quality flag (“A”) in all JHKs filter

bands were used for flux calibration for all of the F2 observations. These three bright standard

stars were consistently within the field of view of all the F2 observations and thus provide robust

calibrators for measuring variability from EM170817. The “A” quality flag indicates 2MASS

photometry with a signal-to-noise ratio (SNR) greater than 10 and a photometric uncertainty

less than 0.109 mag.

Due to its proximity to the core of the host galaxy NGC 4993, photometric measurements

of EM170817 will be contaminated by the extended galaxy emission unless it is properly sub-

tracted. In order to subtract the extended host galaxy emission, we perform a median filter sub-

traction of each image before measuring the flux from EM170817. Median filtering removes

33

small spatial scale features (i.e. point sources) and returns the extended galaxy emission as well

as large spatial scale background structures. We then subtracted the median-filtered image from

the original to remove the extended emission. Since we want to avoid self-subtracting emission

from EM170817 in this process, we set the dimensions of the median filter to ∼ 5 times the

full-width at half maximum (FWHM) of the point-spread function (PSF) as measured from the

2MASS calibrator for each image.

Depending on the SNR of EM170817, aperture or PSF-fitting photometry was performed.

PSF-fitting photometry is preferred due to possible residual emission from the host galaxy

NGC 4993 overlapping with the projected position of EM170817 even after the median fil-

ter subtraction. However, as EM170817 faded the PSF fitting failed to yield robust photometry

(SNR> 5) and thus aperture photometry was used. For aperture photometry, the inner radius of

the aperture was set to the FWHM of the PSF as measured from the bright 2MASS calibrators.

Emission from the sky and residual NGC 4993 background at the position of EM170817 was

estimated from a circular annulus centered on EM170817 with an inner and outer radius of 1

and 2 times the FWHM, respectively.

Palomar WIRC near-infrared imaging

We observed EM170817 on 2017 August 27 in the Ks-band using the Wide Field Infrared

Camera (WIRC; (51)) on the 200-in. Hale Telescope at Palomar Observatory (P200). Due to

the low declination of EM170817 and proximity to the Sun at the time of these observations,

we observed the target before 12-degree twilight at airmass > 7. To deal with the bright and

rapidly varying sky background we used the minimum exposure time for the WIRC detector

of 0.92 s with 8 coadds, allowing us to dither the telescope every ∼ 10 s. We obtained 4 well-

dithered frames before reaching the elevation limit of the telescope, for a total integration time

of 29.44 s. Individual frames were dark-subtracted and flat-fielded. A median sky frame was

34

constructed from the dithered sciences images, scaled to the sky level in each individual frame

and subtracted to remove the bright background. The individual frames were then aligned and

coadded using centroid measurements of 4 well-detected stars, and an astrometric solution was

found by comparing to 2MASS. The photometric zero point was also determined by comparing

aperture photometry of these stars to their 2MASS magnitudes.

IRSF near-infrared imaging

We observed EM170817 on 2017 August 23, 24, 26, and 28 with the near-infrared (JHKs)

simultaneous imaging camera SIRIUS installed on 1.4-m telescope IRSF (InfraRed Survey Fa-

cility) telescope at the South African Astronomical Observatory in Sutherland, South Africa

(PI Barway, Program Barway-2017-01-IRSF-57). We obtained 10 dithered exposures of 30 sec

each with dithering radius of 60 arcsec per observing sequence, respectively. This was repeated

seven to eight times to obtain good SNR. Twilight flat frames were obtained before and af-

ter the observations. Dark frames were obtained at the end of the nights. The data reduction

steps which include dark frame subtraction, flat-field correction, sky-subtraction, dither com-

bination and astrometric calibration were carried out using the SIRIUS data reduction pipeline

software1. Similar to the FLAMINGOS-2 image processing, we perform a median filter sub-

traction on the IRSF images to remove the extended galaxy emission before measuring the flux

from EM170817.

ANDICAM near-infrared imaging

We observed EM170817 on 2017 August 24–28 with the ANDICAM (A Novel Dual Imaging

CAMera) instrument mounted on the 1.3-m telescope at Cerro Tololo Inter-American Observa-

tory (CTIO) in Chile (PI Cobb, Program NOAO 2017B-0160). On 2017 August 24, a total of 16

individual 45 s K-band frames were obtained by imaging at 4 different dither positions at each

1http://irsf-software.appspot.com/yas/nakajima/sirius.html

35

of 4 slightly offset telescope pointings. Each observation from 2017 August 25 to 28 consisted

of a total of 20 individual 90 s K-band frames obtained by imaging at 5 different dither posi-

tions at each of 4 slightly offset telescope pointings. After 2× 2 binning and flat-fielding of the

individual frames, sky frames were produced at each dither position by median-combining all

images taken at the same dither position (but with different telescope pointings). Corresponding

dither position sky frames were subtracted from each image with rescaling to compensate for

variability in background brightness. All sky-subtracted images were aligned and averaged to

produce a single master K-band image for each epoch. Finally, we perform a median filter sub-

traction described in the FLAMGINOS-2 imaging subsection to remove the extended galaxy

emission before measuring the flux from EM170817.

Apache Point Observatory near-infrared imaging

We observed EM170817 in the near-infrared (Ks-band) on 2017 August 27 using the Near-

Infrared Camera & Fabry-Perot Spectrometer (NICFPS; (52)) instrument on the Apache Point

Observatory 3.5-m telescope (PI Chanover, Program 2017 Q3DD04). Observations were ac-

quired towards the end of evening twilight at high airmass (6–8) through highly variable, par-

tially cloudy conditions. Forty 6 s exposures were acquired, alternating between the source

and a position 5′ away using a dither pattern to move the target to different parts of the de-

tector. Because of rapidly varying clouds, only frames showing stars were used in the image

processing. Unregistered frames were combined to form a sky image which was subtracted

from each on-source frame. On-source frames were registered using the Ks = 9.75 magnitude

star 2MASS J13094158−2323149 1.4′ away from NGC 4993 and combined into a final image

using a median filter after adjusting for the variable background level using the mode of each

image. Photometric calibration used the same star as that used for registration.

36

VLT/VISIR mid-infrared imaging

EM170817 was observed using the Very Large Telescope (VLT) spectrometer and imager for

the mid-infrared (VISIR) (53) at the Cassegrain focus of UT3 on 2017 August 23, 2017 Au-

gust 31, 2017 September 01, and 2017 September 062 with the J8.9 filter (central wavelength

8.72µm). Total on-source integration times were 44.8, 17.5, 12.2, and 44.8 minutes, respec-

tively. Chopping and nodding in perpendicular directions with 8′′ amplitudes were used to

remove the sky and telescope thermal background.

Observations of EM170817 and standard stars observed on the same night were reduced

following the imaging data reduction processes outline in the VISIR Pipeline User Manual:

chop and nod frame subtraction, source detection, and shift and add. However, since we did

not detect EM170817 or any other point-source in the field of view, the chopping and nodding

amplitudes and directions provided in the image headers were used to shift the images before

coadding. As a test, we performed the same “blind” shift and add procedure on the images of

the standard stars and found that there were negligible differences in the image quality from

the properly coadded standard star images. Calibration was performed based on mid-infrared

standard stars obtained within same night using the standard-star flux catalog for VISIR imaging

filter based on (54).

Gemini-S GMOS optical imaging

We observed EM170817 in the optical (g- or i-band) several times between 2017 September 1

and 2017 September 5 with the Gemini Multi-Object Spectrographs (GMOS; (55, 56)) at the

Gemini-S observatory. The data were reduced and coadded using the GMOS and GEMTOOL

modules in PyRAF. EM170817 was not detected in any of these data. Limiting magnitudes

(listed in Table S1) were calculated based on aperture photometry with photometric calibra-2These observations were taken under proposal 60.A-9292(B), which the European Southern Observatory

kindly made public to the LV-EM community.

37

tion done relative to the Pan-STARRS PS1 catalogs (57) after utilizing the median subtraction

technique described in the FLAMGINOS-2 imaging subsection.

HST ultraviolet imaging and astrometric referencing

We observed EM170817 in the ultraviolet on 2017 August 22 and 2017 August 29 with the Hub-

ble Space Telescope (HST) Wide Field Camera 3 (WFC3) using the UVIS detectors (PI Kasli-

wal, Program HST-GO-15436). The first epoch used the F225W and F336W filters (2359 A

and 3355 A central wavelengths, the latter similar to U -band) while the second used the F275W

(2704 A central wavelength) and F336W filters. We calculated PSF magnitudes with the soft-

ware package DOLPHOT (v.2.0) (58). EM170817 was only detected in the 2017 August 22

F336W image (see figure S1). For the other images we calculated 5-sigma limiting magnitudes

based on aperture photometry (see Table S1).

We registered the 2017 August 22 stack of F336W images with the Gaia astrometric catalog

(59) to derive a more precise position of EM170817. With 22 matched Gaia sources the refined

astrometric solution has RMS residuals of 0.05′′ and we measure the position of EM170817 to

be (J2000) Right Ascension 13h09m48.071s, Declination −23◦22′53.37′′.

38

Gemini-S FLAMINGOS-2 near-infrared spectroscopy

We observed EM170817 on 2017 August 22 and 2017 August 29 with the FLAMINGOS-2

spectrograph (49) on the 8.1-m Gemini-S Telescope on Cerro Pachon in Chile. For the first

epoch we used the 3-pixel (0.54′′ wide) slit and obtained spectra with both the JH and HK

grism/filter sets to achieve a spectral resolution of R = 600 across the 1.0–2.4µm spectral

range. Using an exposure time of 150 s, we obtained a sequence of 8 frames in the HK setup

and 6 frames in the JH setup. The target was nodded along the slit between frames in an ABBA

pattern to allow for accurate subtraction of the sky background. Baseline calibrations were also

obtained, including dark frames, spectral flat fields, Ar arc lamp spectra for wavelength calibra-

tion, and observations of the A0V star HIP 69718 immediately after the science observations at

similar airmass. For the second epoch we increased the total integration time to 16 frames of

150 s each using the HK setup.

Dark current subtraction, flat-fielding, sky background subtraction, coadding of the 2D spec-

tra, wavelength calibrations, and 1D extractions were performed using standard tasks in the

Gemini IRAF3 package. Telluric corrections and flux calibrations were performed using the

IDL tool xtellcor (60). A model spectrum of Vega was used to match and remove the hydrogen

lines of the Paschen and Brackett series from the A0V telluric standard and construct a telluric

correction spectrum free of stellar absorption features. The resulting telluric correction spec-

trum was also used for the initial flux calibration. The absolute flux calibration was then found

by comparing synthetic photometry derived from the spectra to the H-band photometric mea-

surement of the transient at the same phase. Note that in the second epoch, despite the increased

exposure time we did not detect EM170817.

3IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Associationof Universities for Research in Astronomy (AURA) under a cooperative agreement with the National ScienceFoundation.

39

Gemini-S GMOS optical spectroscopy

We observed EM170817 on 2017 August 20 and 2017 August 21 using the Gemini Multi-Object

Spectrographs (GMOS; (55,56)) with R400 and B600 gratings. The spectra were reduced using

the Gemini IRAF package4. The standard star (EG274) observed on 2017 August 20 was used

to flux-calibrate spectra from both of the observed epochs. Both spectra show largely featureless

continua. The observations taken on 2017 August 21 show a clear trace on both the blue and

red sides of the detectors. The trace on the blue side of the observation on 2017 August 20 was

too weak to extract.

Keck LRIS optical spectroscopy

We observed EM170817 on 2017 August 25 using the Low-Resolution Imaging Spectrometer

(LRIS; (61)) on the 10-m Keck I telescope. The visibility of the target position to Mauna

Kea was poor and the observations were conducted under non-optimal conditions at the end of

evening twilight, with the airmass in the direction of the target increasing from 3.8 at the start of

the sequence to ∼6 at the end of the final exposure. We used the D560 dichroic to split the light

between the red and blue sides, and used the 400/8500 grating on the red side and the 400/3400

grism on the blue side. The observations were processed using our analysis pipeline LPIPE and

summed. A weak trace is visible at the transient location on the red-side camera. Our (low-

S/N) extraction of the spectrum (figures S2 and S3) shows a featureless red continuum with no

significant features (in particular, no broad or narrow emission lines), although we note that a

clean extraction is complicated by the significant contamination from the spatially-varying host

galaxy continuum.

4http://www.gemini.edu/sciops/data-and-results/processing-software

40

Bolometric Light Curve Construction

We combined available ultraviolet, optical, and near-infrared photometric data (including our

Gemini, Keck, HST, IRSF, ANDICAM, and P200 data along with data published in LVC GCNs

and (11); Table S1)5 on EM170817 to build a bolometric light curve using two independent ap-

proaches illustrated in figure 2. All analysis used a distance of 40 Mpc (66,67) and a reddening

of E(B − V ) = 0.1 mag (68).

Our first method uses low-order polynomials to enable interpolation of the photometry in

the individual filters. A blackbody is then fitted to the interpolated photometry for a set of dis-

crete epochs starting from 0.49 d after GW170817 (0.04 d after the first i-band detection) up

until 12.9 d after GW170817. Second, we fit the data with an evolving blackbody model by

assuming a functional form for the time-dependence of the blackbody temperature T (t) and the

radiusR(t). After experimenting with different models we found the best functional forms to be

R(t) = R0(1− e−λt) + vt (a model that initially decelerates and then coasts) and T (t) = T0tα.

The data were fit by computing the monochromatic flux density as a function of time and wave-

length appropriate for each observation (corrected for extinction assuming AV = 0.31 mag, and

using the reddening law of (69) when no instrument-specific reddening data was available) and

comparing with the data; no interpolation or binning was used on the data themselves. The

parameter fitting was done using a Markov Chain Monte Carlo method (70): we used 80 walk-

ers, fit for 50 iterations to achieve “burn-in”, and then fit for a further 1000 iterations, only

using every 41st value to account for finite autocorrelation in the resulting chains. We then

determined the best-fit parameter values and uncertainties from the posterior distributions. We

restricted the fit to times ≤ 12.5 d after GW170817, since after that time the multi-wavelength

coverage was significantly diminished and we typically only had detections in a single band

5 Note that we did not use photometry from several telescopes where the results reported in LVC GCNs wereuncertain (62–65).

41

(Ks). Our best-fit values and uncertainties (68% confidence limits) are R0 = 24000± 3000R�,

v = 2500 ± 300R� d−1 = 20000 ± 3000 km s−1, λ = 0.42 ± 0.05 d, T0 = 6050 ± 50 K,

and α = −0.62 ± 0.01, with t in days. A model with λ finite (i.e., initially decelerating) is

preferred over a model with constant velocity: χ2 is reduced by 600 with roughly 140 degrees-

of-freedom. Both approaches give us consistent results for effective temperature, photospheric

radius and kinematics (see main text and figure 2). Note that in neither case do we compute

a formal goodness-of-fit, as our data-set is too inhomogeneous. We are drawing data from

24 different telescopes, with many photometric systems, filter transformations, extinction co-

efficients, and zero-points, not all of which are fully documented. There are also significant

variations in calibration methodology and host-galaxy subtraction, all of which can affect the

resulting photometry. We have manually removed the data where host subtraction was known

to be problematic, but quantitative model evaluation (e.g., for deviations from blackbodies) will

have to wait for a uniform re-analysis of all of the data.

42

Supplementary Text

Census of the Local Universe Galaxy Catalog

In this section we describe the galaxy catalog used to narrow down our search of EMGW can-

didates within the GW trigger volume and provide basic and physical properties for each of the

galaxies within the 90% volume. We find that the galaxies in the 90% volume span a large range

of properties containing low-mass dwarfs as well as larger galaxies (spirals and ellipticals).

The forthcoming Census of the Local Universe (CLU) galaxy catalog consists of two parts:

a compilation of known galaxies from many sources; and new galaxies (i.e., those with no pre-

vious distance information) from a four-filter, narrowband survey designed to find redshifted

Hα emission out to 200 Mpc (based on conservative predictions for the horizon of NS-NS de-

tections with aLIGO/Virgo (71, 72)) across ≈ 3π of the northern sky (7). However, the GW

localization was confined to a declination (near −25◦) below the southern limit of the narrow-

band survey (δ ≥ −20◦), so only the compiled galaxy catalog overlapped with the GW event

and will be described here.

The compiled portion of the CLU galaxy catalog (73) was taken from existing galaxy

databases: NASA/IPAC Extragalactic Database (NED)6, Hyperleda7 (74), Extragalactic Dis-

tance Database8 (EDD; (75)), the Sloan digital sky survey DR12 (SDSS; (76)), and The Arecibo

Legacy Fast ALFA (ALFALFA; (77)). The catalog contains ∼260,000 galaxies with existing

spectroscopic distances less than 200 Mpc. The current version of the catalog contains updates

from both NED and SDSS in early 2016. Distances based on Tully-Fisher methods were favored

over kinematic (i.e., redshift) distances because of the large contribution of peculiar velocities

in this local sample; however, the majority of the distances are based upon redshift information.

6https://ned.ipac.caltech.edu7http://leda.univ-lyon1.fr8http://edd.ifa.hawaii.edu

43

In addition to distances, the catalog also contains compiled multi-wavelength photomet-

ric information. We have cross-matched the CLU catalog with GALEX all sky (78), WISE all

sky (79), and SDSS DR12 (76) surveys to obtain fluxes from the ultraviolet (UV) to the in-

frared (IR), with ∼104,000 matches for GALEX FUV, ∼226,000 for WISE 3.4 and 22µm, and

∼114,000 for SDSS r-band.

We have spatially cross-matched our CLU galaxy catalog to the 90% containment volume

of the LIGO/Virgo trigger (5) reported by the BAYESTAR probability sky map (80) with no

assumption on orientation and found 49 galaxies. In addition, we have used the GALEX FUV

and WISE 3.4 and 22µm bands to measure the physical properties of these galaxies. The star

formation rates (SFRs) are derived from GALEX FUV Kron-magnitudes (81) after correcting

for Milky Way extinction (68) as tabulated by NED and internal dust extinction using a com-

bination of observed GALEX FUV and WISE 22µm fluxes (82). The stellar masses (M?) are

derived from 3.4µm ALLWISE fluxes and a mass-to-light ratio of 0.5 (83).

Table 3 lists the basic properties of the 49 galaxies (sorted by stellar mass) along with the

UV and IR fluxes, SFRs, stellar masses, and the probabilities in the containment volume (84).

The galaxies span ranges of stellar mass (107M� to 1010.5M�) and SFR (10−2.6M� yr−1 to

100.5M� yr−1) suggesting that the list contains dwarfs as well as larger more massive galaxies.

44

Host Galaxy NGC 4993Basic Properties

The EM counterpart for GW170817, EM170817, was reported near NGC 4993, the third most

massive of the galaxies cross-matched to the LIGO/VIRGO trigger (note that NGC 4993 was

not present in the top 15 galaxies in the GLADE catalog (85) and reported in (86), although it

is present in their on-line catalog). NGC 4993 has been classified as an E-S0 with a morpho-

logical T-type of −3 (87) at a distance of roughly 40 Mpc, based on Tully-Fisher measurements

of other galaxies in its group (66, 67). The archival HST/ACS F606W image (Program ID:

14840; PI Bellini) shows complicated dust lanes extending a few kpc from the nucleus (88, 89)

superimposed on an elliptical galaxy, similar to many other early-type galaxies. The dust lanes

may be the result of a minor merger that occurred as long as several Gyr ago. Note that the

dust lanes do not extend across the position of EM170817 (figure S1), suggesting EM170817 is

not located in a heavily-obscured region. This is echoed by examination of spatially-resolved

spectra, which show no emission-lines within ±5′′ (±1 kpc) of the transient, and our HST UV

images which show very little emission at the position of the EM170817 (Figure S1).

Archival optical spectra (90, 91) show a continuum dominated by old stars with a pro-

nounced 4000 A break indicative of little if any active star formation; [NII], [SII], and weak

[OIII] emission lines are present in the nucleus with likely some Hα emission filling in the

Balmer absorption from the stellar continuum, but the relatively high [NII]/Hα ratio is sugges-

tive of a low-luminosity active galactic nucleus (LLAGN) rather than star formation. Despite

the lack of ongoing star formation or much nuclear Hα emission, we note that NGC 4993 would

have been detected in our ongoing Hα narrow-band imaging survey (section ) just via the [NII]

emission (92): we estimate a flux for NGC 4993 of ∼ 10−12 erg s−1 cm−2, which is consid-

erably higher than our Hα sensitivity threshold (estimated at 90% completeness at a flux of

10−14 erg s−1 cm−2).

45

We estimated the Eddington ratio by computing the bolometric luminosity from the X-

ray luminosity (5.6×1039 erg s−1; (93)) assuming Lbol/LX−ray = 16 for LLAGNs (94), while

the Eddington luminosity LEdd is determined from black hole mass obtained from the central

velocity dispersion (163 km s−1; (90)); these imply Lbol/LEdd = 1.4×10−5, which is similar to

ratios of other LLAGNs (94).

Constraints on the NS-NS Merger Timescale

To further constrain the timescale of NS-NS mergers, we investigated star formation histories of

NGC 4993 by fitting the spectral energy distribution (SED) with the package MAGPHYS9 (95).

MAGPHYS uses the stellar population syntheses code of (96) and (97) to provide spectral

evolution at wavelengths from 912 A to 1 mm and at ages between 1 × 105 and 2 × 1010 yr.

We collected photometric data from the GALEX (NUV and FUV; (78, 98)), Pan-STARRS1

(grizy; (57)), 2MASS (JHKs; (50)), WISE (W1, W2, W3, W4; (79)) and IRAS (60µm; (99))

surveys, where we used upper limits for the GALEX/FUV and IRAS/60µm bands (Figure S7).

The best-fit model gives the stellar mass of the galaxyM? ∼ 3×1010M� and the star formation

rate (SFR) ∼ 3 × 10−3M� yr−1. Note that there appears to be an offset between the data

and the model in the region near the near-infrared (2MASS JHKs) and mid-infrared (WISE)

observations. This may reflect different methods of measuring the entire extent of NGC 4993

used for the different catalogs. Further examination of the archival images (rather than just

photometric catalogs) will help resolve this issue.

For comparison, the SFR derived from GALEX/FUV is ∼ 4× 10−2M� yr−1, the SFR from

GALEX/NUV for stars more massive than 5M� is∼ 3×10−3M� yr−1 (100,101), and the SFR

from the IRAS/60µm upper limit is < 0.08M� yr−1 (102, 103). Given the varying systematics

and uncertainties in both the data and the methods, we consider these to be largely consistent

9http://www.iap.fr/magphys/

46

and indicative of small levels of ongoing star-formation, with SFR of 10−3 to 10−2M� yr−1.

This is consistent with estimates based on the non-detection of neutral hydrogen (104). It should

be noted that the estimation of the SFR is an upper limit because of the possible contamination

of the central AGN.

The time since the last burst of star formation ended given by the best-fit model is ∼2 Gyr,

implying a long timescale for the merger for the NS-NS binary system. Even a more conser-

vative limit based on the absence of early-type stars in the spectrum puts the last episodes of

star-formation more than a Gyr ago. This is not consistent with previous predictions that the

delay time of NS-NS mergers is short (1 – 100 Myr; (105, 106)), although there are Galactic

NS-NS binaries with merger times 100 Myr–10 Gyr (e.g., (107)). Moreover, population synthe-

sis of compact object mergers finds significant numbers of sources produced after Gyr delays,

with the progenitor systems formed at high redshift during the peak epochs of star formation

(e.g., (108)).

47

Models

In the sections below, we discuss several details of models to explain the overall electromagnetic

emission from EM170817. We estimate the maximum ejecta mass possible to have a weak, on-

axis, ultra-relativistic jet break out and find a number that is implausibly low. We then consider

a model where the observed γ-rays are produced by an off-axis short γ-ray burst (sGRB), and

find it very difficult to produce the observed properties. We consider a structured jet and discuss

the challenges with this model. We rule out a Newtonian source based on the high opacity. This

motivates us to describe the analytical and numerical details of our preferred model: a shock

breaking through a cocoon of material. Finally, we consider alternative sources to power the

early-time emission and find that an engine-driven wind is plausible but free neutron decay is

implausible.

A Weak On-Axis Jet

The observed γ-rays carried an isotropic equivalent energy of ∼ 3 × 1046 erg over a duration

of ∼ 2 s, corresponding to an average luminosity of ∼ 1.5 × 1046 erg s−1 (3, 4). Short GRB

jets produce γ-rays very efficiently (e.g., (32)), so if we observe a regular on-axis GRB the total

jet isotropic equivalent energy is at most a few times larger than that of the observed γ-rays.

Therefore, to evaluate if the observed γ-rays could have been produced by a weak on-axis jet we

estimate the maximum ejecta mass that a jet with an opening angle θj and isotropic equivalent

luminosity 1047 erg s−1 can cross within 2 s. We approximate the ejecta as being static and

spherical, with a density profile ρ ∝ r−2, where r is the radius. We assume a typical ejecta

velocity of 0.2c, a breakout time of 2 s and r = 1010 cm. We use the jet propagation model

of (109) which was calibrated numerically by (110) to estimate the breakout time10. We find

that for θj = 10◦ a breakout of 2 s is achieved for mass < 3× 10−6M�. For θj = 30◦ the upper

10An applet that calculates the breakout time for various configurations can be found athttp://www.astro.tau.ac.il/ore/propagation.html

48

limit on the mass is lower by about an order of magnitude.

To verify this calculation we carried out a numerical simulation where a jet is launched into

an expanding ejecta. The delay between the merger and the jet launch was 0.1 s (a short delay

decreases the breakout time), the jet isotropic luminosity was 1047 erg s−1, and its opening angle

was 8◦. We varied the total mass until the breakout took place at 2 s, which happened near a

mass of 2.5× 10−6M�, very close to the analytic prediction.

Observational and theoretical considerations indicate that the amount of mass that was

ejected at high latitudes along the jet path is higher than 10−5M� by orders of magnitude. First,

the bright optical emission during the first day implies that about 0.02 M� with a relatively low

optical depth (κ ≈ 1 cm2 g−1) were ejected. This type of material is expected to be synthesized

after the merger in the high latitude wind that is exposed to a high neutrino flux (e.g., (111)).

Second, optical/IR emission indicates that the total ejected mass is at least 0.05 M�, and while

this mass is most likely not distributed isotropically, numerical simulations show that all mass

ejection processes throw a non-negligible fraction of mass at high latitudes (e.g., (25,111,112)).

Moreover, all these simulations find the that the high latitude ejecta mass is larger than about

10−3 M�. We therefore conclude that the observed γ-rays are highly unlikely to be produced

by an on-axis low-luminosity jet.

An Off-Axis Jet

An intriguing potential explanation for the extremely low luminosity of the observed γ-ray

emission of EM170817 is that we observe a regular luminous sGRB, but our line-of-sight is

outside of the GRB jet and the low luminosity is due to the lower Doppler boost compared to

an on-axis observer. Below we examine the implications of such a configuration.

Consider a jet with an opening angle θj and a Lorentz factor Γ � 1 that radiates γ-rays.

An on-axis observer sees γ-ray emission with a total energy E, a typical photon energy Ep and

49

a total duration t. We are interested in what an off-axis observer at a viewing angle θobs > θj

will see. We define the quantity q = (θobs − θj)Γ since for an off axis observer q � 1 and the

Doppler boost ratio to an on-axis observer is∝ q−2. We denote all the off-axis observables with

prime.

The effect of the Lorentz boost on the photons’ energies implies:

EpE ′p

= q2 (S1)

The observed total isotropic equivalent energy (fluence) ratio has three regimes, depending on

how far away the off-axis observer is from the edge of the jet and how wide the jet is compared

with 1/Γ:

A ≡ E

E ′=

q4 ; θobs − θj � θj (i)

q6(θjΓ)−2 ; θobs − θj � θj > 1/Γ (ii)

q6 ; θj < 1/Γ (iii)

(S2)

where we define an amplification parameter A ≡ E/E ′. To understand this equation it is most

convenient to consider first the last case, θj < 1/Γ, since then the jet can be regarded as a point

source. If θobs − θj � θj > 1/Γ then the whole solid angle of the jet (θ2j ) contributes roughly

equally to the observed fluence for an off-axis observer, compared to a solid angle 1/Γ2 that

dominates the emission for an on-axis observer, thereby reducing the fluence ratio by a factor

of (θjΓ)2. Finally, when θobs − θj � θj the whole jet does not contribute equally to an off-axis

observer and the emission is dominated roughly by a solid angle of (θobs − θj)2.

The observed duration depends on the details. If we assume that each observed pulse has a

duration δt and that it is generated by an episode of emission that takes place at a radius r over

some radii range ∆r ∼ r then δt ≈ r/(2cΓ2) and

δt

δt′= q−2 (S3)

If the total on-axis duration is determined not by a single episode, but by the radial length of the

50

jet (as in the case of internal shocks for example) then

t′ = max{q2δt, t} (S4)

For EM170817 E ′ ≈ 3 × 1046 erg (3, 4). If the on-axis observer sees a regular sGRB then

A = 103 − 106 with a typical value of A = 104. Note that A = 103 may be too low due

to the requirement that the jet breaks out of the ejecta within 2 s, as a jet with a luminosity of

1050 erg s−1 can break out on time only if the ejecta mass along its path is < 10−3M� (isotropic

equivalent). This is much lower than the total mass we observed assuming an opacity κ ≈

1cm2 g−1 of ∼ 0.02M� (see main text), which is ejected presumably at high latitudes.

Classical sGRBs show non-thermal spectra and therefore the observed photons are expected

to be generated above or near the photosphere. This expectation was one of the main indications

that GRB jets are relativistic (113, 114) and it still provides the most robust lower limit on the

Lorentz factor of GRB jets. The optical depth of a relativistic jet that radiates a spectrum with

a power-law and exponential cutoff, Nν ∝ να exp[hν/E0], such as the one seen in EM170817,

is given by (31):

τΓ ≈ 1013L51δ−1t,−2

mec2

E0

Γ−(4−α) exp

[−Γmec

2

E0

](S5)

where L is the burst luminosity, mec2 is the electron rest-mass energy and we use the notation

Qx = Q/10x in c.g.s. units. This expression can be written using the off-axis observables and

A:

τΓ ≈ 107AL′47δ′−1t,0

mec2

E′0Γ−(4−α) exp

[−Γmec2

E0

](S6)

Note that there is still a dependence on the on-axis observedE0 in the exponent of this equation,

for which the transformation to the off-axis frame depends on A differently for each of the three

regimes in equation S2. Therefore the limit on Γ that we derive below is different for each

regime.

51

The requirement that the γ-ray source is optically thin, i.e., τΓ < 1, provides a lower limit

on the jet Lorentz factor Γ(A), which in turn determines the maximal distance that we can be

away from the edge of jet (i.e., θobs − θj) for each value of A. Figure S8 shows that distance

for cases (i) and (iii) which are independent of θj. The results for case (ii) are similar for all

realistic values of θj. This suggests that for θobs ≈ 0.5 rad we can never be in the far regimes,

i.e., (ii) or (iii) since θobs − θj � θj is always true. Figure S9 shows the minimal Lorentz factor

of the jet only for case (i) (near miss). This implies that if the observed γ-rays are generated by

an off-axis jet then it must have Γ > 100 and the angle between us and the jet’s edge cannot

exceed 0.1 rad.

A luminous jet that we miss by only 0.1 rad is expected to produce a very bright afterglow

roughly a day after the burst. The reason is that the blast wave driven into the circum-merger

medium, which produces the afterglow, decelerates to a Lorentz factor < 10 by that time and

thus its cone of emission gets into our line of sight. Hence, after about 1 d we are expected

to see a regular sGRB afterglow. As shown in (11, 13) our limits on the X-ray and radio rule

out this option. The only way to avoid a bright afterglow is if the merger took place in a very

low density environment. In fact, taking the minimal Lorentz factor allowed by compactness

(i.e., the maximal distance to the jet edge), and a typical sGRB energy with A = 104, the

circum-burst number density should be as low as∼ 10−6 cm−3 for the afterglow to be consistent

with the observed limits and detections in the X-ray and the radio. This value is lower by

orders of magnitudes than the density inferred by sGRB afterglows (115) and for this specific

event it is unexpected given the modest offset of the merger location from the center of its

host (although we do not know the offset along the line-of-sight). Moreover, based on neutral

hydrogen mass functions, a density of 10−6 cm−3 is ten times less likely than 10−3 cm−3 in early

type galaxies (116). In addition, if EM170817 were an off-axis sGRB, then an on-axis observer

would have seen a burst with a typical photon energy of Ep ≈ 10 MeV, much higher than the

52

values observed in sGRBs (117,118). Finally, the probability to have such a near miss of the jet

is only a few percent. In fact the chance to see a regular on-axis GRB is significantly higher in

that case, since the jet is relatively wide (≈ 0.4 rad).

To conclude, in order for the EM170817 to be an off-axis sGRB, in addition to being lucky,

we need to push all the parameters to their limits. The Lorentz factor would be the lowest

one allowed by the compactness limit and the surrounding density would be extremely low. In

addition the on-axis sGRB should have an Ep that is much higher than that observed in sGRBs.

This implies that it is unlikely that EM170817 is generated by an off-axis observation of a

typical sGRB.

A Structured Jet

We consider the possibility that short GRB jets have structure – a core with high-luminosity and

high Lorentz factor; wings with lower luminosity and/or lower Lorentz factor. This structure

may be induced by the jet launching mechanism or by to the interaction of the jet with the

ejecta. The question then is whether the observed γ-rays could have been generated at the jet’s

wings, either on-axis or off-axis. Below we consider both options.

If we observe the wings on-axis, then these are wide-angle low-luminosity wings. It is

unclear how such wings would be generated. If a jet is launched with a high luminosity narrow

core and low-luminosity wide angle wings, then the situation is similar to the on-axis wide and

weak jet discussed above. The propagation of the narrow core does not facilitate the propagation

of the wide low-luminosity wings, which will be choked by the ejecta. The interaction with the

ejecta is also unlikely to produce low-luminosity wide wings because a cocoon, which has a

comparable energy to the jet itself, is expected to dominate the wide angle outflow.

Off-axis emission from moderate Lorentz factor (say Γ ∼ 10) material at the jet wings is

more plausible. In this scenario the lower Lorentz factor of the jet wings allows the γ-rays to

53

be observed although the angle to the jet edge is large enough so the afterglow emission will

not violate the X-ray and radio observations. However, this configuration is also in tension with

the observations. First, the compactness criterion that we used in order to constrain the off-

axis emission (see above) shows that material with low Lorentz factor that is observed off-axis

cannot have a high amplification factor A. Namely it must have low luminosity. For example a

source with Γ ≈ 10 is limited to A ≈ 20, or a total isotropic equivalent energy of ∼ 6 × 1047

erg as seen by an on-axis observer. Such low energy material is again expected to be suppressed

by the cocoon. Moreover, observations of short GRBs do not support a structured jet with a

luminosity that gradually drops at the wings. The reason is that with such a structure we would

expect to detect at least some sGRBs where the γ-rays are generated by the low-luminosity

wings that point towards us. The afterglows from these GRBs would have shown after a day or

so the signature of an off-axis jet that carries significantly more energy than the one observed in

γ-rays, once the the emission becomes dominated by the high energy core. Such short GRBs are

not observed (see for example a compilation of sGRB prompt and afterglow properties by (32)).

We conclude that while in principle the observed γ-rays may have been generated by low-

luminosity low-Lorentz factor wings of a jet seen off-axis, current observations and theory

disfavor this possibility.

A Newtonian γ-ray source

Assume that the γ-ray source is Newtonian. The optical depth of the source, due to produced

pairs, can be estimated using equation S5 with L51 = 10−5 − 10−4 (3,4)and δt,−2 ≈ 100. Thus,

if Γ = 1 the source is extremely opaque. If we assume the maximal possible radius given the

burst duration ∼ 3× 1010 cm, the lower limit on the optical depth at the source is ≈ 105 − 106.

Such a large optical depth is unrealistic for many reasons. For example, the observed spectrum

below Ep is much shallower (softer) than a blackbody, while the spectrum of such a source is

54

expected to be a blackbody or a Wein (3,4). Another problem is the implied diffusion time. The

burst duration must be shorter than the diffusion time, which with this optical depth implies that

the width of the emitting region cannot exceed ∼ 105cm during the entire emission. Such a

narrow non-expanding emitting region is not expected in ejecta that expands at sub relativistic

velocities at a radius of ∼ 3× 1010 cm.

Thus, we conclude that the γ-ray source must have a low optical depth. Equation S5 shows

that Γ = 2− 3 (mildly relativistic) is sufficient for an optically thin source.

A Mildly Relativistic Shock Breakout of a Cocoon

When a fast shock propagates in a medium with a high optical depth it is dominated by radiation.

The shock breaks out once the optical depth drops so it cannot confine the radiation within the

shock layer and it escapes to infinity. The theory of relativistic shock breakout was developed in

the context of a shock that propagates in a star (119,120) where the unshocked material is static

and the stellar structure dictates its propagation. In the scenario of a breakout from the ejecta of

a binary neutron star merger the shock breaks out of expanding material, which has a different

density profile than a star. Nevertheless, since the pulse of radiation that is emitted upon the

shock breakout is dominated by the radiation generated within a very thin layer with an optical

depth of unity, its properties depend almost entirely on two physical parameters: the shock

Lorentz factor, Γbo, and the breakout radius, Rbo. Thus, we can use the derivation of (120) to

test whether a shock breakout can produce the observed signal, and if it does then to estimate

its properties.

Not every flare of γ-rays can be generated by a relativistic shock breakout. First, shock

breakouts do not produce flares with a highly variable temporal structure. This by itself implies

that almost all GRBs (short and long) cannot be generated by shock breakouts. Second, the

three main observables of the flare: energy, Ebo, duration tbo and temperature, Tbo (note that

55

the spectrum is not expected to be a blackbody, and Tbo is just the typical photon energy),

depend only on two physical parameters, Γbo and Rbo. Therefore they must roughly satisfy a

closure relation (equation 18 in (120)):

tbo ∼ 1 s

(Ebo

1046 erg

)1/2(Tbo

150 keV

)− 9+√3

4

(S7)

The flare of γ-rays that followed EM170817 (3, 4), which released Ebo ≈ 4 × 1046 erg over

a duration of tbo ∼ 1 − 2 s at a typical photon energy of Tbo ∼ 100 − 150 keV, satisfies the

relation. Again, almost all the regular GRBs (short and long) do not satisfy this relation. They

are too energetic and soft for their duration. The only type of GRB that satisfies equation S7 and

shows a non-variable light curves are low-luminosity GRBs, for which we have strong evidence

that the γ-rays are generated by a mildly relativistic shock breakout (121).

Using any two of the three equations 14, 16 17 from (120) we can find that if a shock

breakout is the source of the γ-rays that followed GW170817 thenRbo ∼ 3×1011 cm and Γbo ≈

2 − 3. Note, that equation S7 and the estimates of the breakout parameters both assume that

the shock breakout takes place over a relatively wide angle (> 20◦) that includes the observer’s

line-of-sight. This implies that in the case of EM170817 the cocoon breakout must have taken

place over a wide angle of at least 0.5–1 rad if it was the source of γ-ray emission.

Hydrodynamical Simulation Details of A Cocoon Breakout

To verify that a model of a cocoon driven by a choked jet can explain the full range of electro-

magnetic observations, we carry out 2D relativistic hydrodynamic simulations, followed by a

post-processing calculation of the UV/Optical/IR emission during the expansion of the cocoon

and the ejecta. We search for a model in which the delay between the merger and the observed

photons from the breakout is 2 s (3,4) and that its breakout radius and velocity match those that

we calculate above. Note that we do not directly calculate the γ-ray signal since the breakout

56

takes place over a scale that is much smaller than the scales that our simulation can resolve.

However, the model is required to produce UV/Optical/IR signal that fits the observations.

For the 2D relativistic hydrodynamic simulations we use the public code PLUTO (122). The

initial configuration at t = 0 (defined as the merger time) is cold ejecta that expands radially,

which is present from the base of the grid at resc = 4 × 108 cm up to rmax = 2 × 109 cm. The

ejecta have an angular profile, where most of the mass (75%) is near the equator at θ > 1.0 rad,

where θ is the polar angle. The ejecta are also divided in the radial direction into two regions

(with similar angular profiles) – a massive slow core that extends at t = 0 up to rc = 1.3×109 cm

and low-mass fast material that extends at t = 0 between rc and 2× 109 cm. The density profile

of the dense core is:

ρcore(r, θ) = ρ0r−2

(1

4+ sin3 θ

), (S8)

where ρ0 is the normalization which is chosen to fix the total ejecta mass. The velocity profile

of the core is

vcore(r) = vc,maxr

rc, (S9)

where vc,max = 0.2c is the maximal velocity of the ejecta’s inner part. The extended ejecta

density profile is chosen as a very steep power-law in v between vc,max and 0.8c so its total mass

is 1% of the core mass and the mass carried by material at v > 0.7c is about 10−5 of the total

ejecta mass. The jet is injected into the ejecta with a delay of 1 s for a total working time of 2 s

and a total luminosity of Lj. The jet is injected with a specific enthalpy of 20 at an opening angle

of 0.7 rad from a nozzle at the base of the grid with a size of 108 cm. We search for a model that

fits the evolution of the bolometric luminosity and temperature fitted to the UV/optical/IR data

by varying the total ejecta mass, namely ρ0, and the jet’s luminosity, Lj. We find reasonable

fits with ejecta masses larger than 0.05M� and jet luminosities that release an energy that is

comparable or a bit larger than the total ejecta kinetic energy. The specific model presented in

the paper (figs 2 and 6) and which is discussed below has a total ejecta mass of 0.1M� and jet

57

luminosity Lj = 2.6× 1051 erg s−1.

Given its large opening angle the jet is not collimated. Instead it works its way through a

large amount of ejecta dissipating its energy in the process, forming a cocoon. The jet injection

stops at 3 s and by t ≈ 4 s the jet is fully choked, leaving a hot cocoon that continues to

propagate driving a mildly relativistic (Γ ≈ 2 − 3) shock into the ejecta. About 10 s after

the merger the shock catches up with the leading edge of the ejecta and breaks out at a radius

of Rbo = 2.4 × 1011 cm = 8 light-seconds. Thus, photons released during the cocoon shock

breakout are only 2 s behind the gravitational waves released at the merger, thereby consistent

with the delay between the merger and γ-rays observed by Fermi (3,4). The breakout also takes

place at a radius and velocity that is expected to produce a signal consistent with EM170817.

We end the simulation at t = 15 s when all the expansion is homologous and all the material

moves ballistically. The emission after the breakout is calculated as a post-processing of the

final snapshot of the hydrodynamical simulation, following a similar procedure to that described

in (29). It contains two components: (i) diffusion of photons that where deposited by the shock

that crossed the ejecta, which we call “cooling emission”, and (ii) radioactive decay of the

elements that where synthesized in the ejecta, termed “macronova”. In short, for each time

step in the lab frame we calculate first the radial optical depth from every radius to infinity

along a radial path. We do that for different angles and we determine the trapping radius,

rt(θ), where the optical depth τ(rt(θ)) = c/v. Above this radius, photons diffuse freely to the

observer at infinity while below this radius they are trapped. This is an approximation since

the outflow is not spherically symmetric. In a similar manner, we calculate for each angle the

photospheric radius rph(θ) for which τ(rph(θ)) = 1. To obtain an approximated calculation that

includes correction due to the mildly relativistic motion, we first calculate the luminosity in the

comoving frame. The cooling emission luminosity in a comoving frame is determined by the

diffusion of the rest frame energy flux at the trapping radius of the radiation that was carried by

58

the outflow from the last hydrodynamical snapshot. The macronova emission arises from the

radioactive heating generated by all the material above the trapping radius, m(r > rt), and is

estimated at the rest frame of the photosphere as ε = m(r > rt)ε01010(t

1 d

)−1.3erg s−1, where

ε0 is of order unity (123). Following (111) we take ε0 = 2 during the first two days and reduce

it to ε0 = 1.5 afterwards.

Next, we estimate the rest frame temperature at rph(θ) by taking for each direction the rest

frame luminosity (the sum of cooling and macronova emission) and finding the radiation energy

density at the photosphere assuming that the radiation is in thermal equilibrium and that the

local radiation spectrum is a blackbody at this point. We assume that this blackbody radiation

is emitted isotropically at the matter’s rest frame of the photosphere. Having the rest frame

luminosity and spectrum along each angle at every time in the explosion frame, we integrate

the contribution from material at all angles for observers at different viewing angles at different

observer times, by properly accounting for the Lorentz boost and the light travel time.

Finally, in order to find the ejecta optical depth at each time and place we need to estimate

its opacity. For that we use tracers which mark material that is ejected initially (at t = 0) at

different angles. By looking at the velocity distribution of the tracers at the end of the simulation

we find that material that was ejected initially near the equator is associated with the slowest

velocities (v/c < 0.1) while material that was ejected initially at high latitudes is associated

with faster material. This is not surprising given that the energy of the jet was mostly deposited

at high latitude material. Following (111) we adopt an opacity κ = 10 cm2 g−1 for material

slower than 0.1c, which is presumably dominated by neutron rich dynamical ejecta that contains

Lanthanides, and κ = 1.0 cm2 g−1 for material that is faster than 0.1c which was presumably

ejected from the disk and/or the massive neutron star that were formed after the merger and

does not contain r-process elements from the second and third peaks.

Throughout the simulations we applied an ideal gas equation of state with an adiabatic

59

index of 4/3 as appropriate for a radiation-dominated gas, and neglect gravity given that kinetic

energy dominates the ejecta in the simulation domain. For the integration an HLL Riemann

solver and a third order Runge Kutta time-stepping have been used. We employed a cylindrical

grid with 1620 × 1600 cells. The grid is divided in three patches on the x-axis and two on

the z-axis (parallel to the jet axis). The innermost patch on the x-axis is stretching from the

origin to x = 2 × 108 cm with 20 cells uniformly distributed, this patch makes sure the jet’s

nozzle contains enough cells. Most of the cocoon forms and experiences the mixing up to

x = 2 × 1010 cm and z = 2 × 1010 cm. In this region we employ a resolution of 800 cells

on each axis. Outside of these coordinates, the grid contains a uniform distribution of 800

additional cells in each axis up to x = 4× 1011 cm and z = 4× 1011 cm.

Early-time emission powered by an engine-driven wind

In the main paper we proposed two options to explain bright and blue emission seen on the first

day. Both options rely on radio-active heating (one involved mildly relativistic material and

the other material with relatively low opacity) to power the emission. Here we suggest a third

possibility: the release of internal energy that was deposited in the ejecta by an engine-driven

wide-angle wind that lasted for ∼ 20s.

In the cocoon model that we presented above, the cooling emission fades within hours and

does not contribute to the observed emission. The energy is deposited by the jet over a short

duration when the ejecta radius is small. Then adiabatic losses cool the radiation by the time

that the radiation is emitted. This can be shown using the following simple arguments. Assume

that energy is deposited at radius R0 in a mass M . This energy is deposited by a strong shock

so about half of it is deposited in the form of radiation while the other half accelerates the mass

to a velocity v = βc. The radiation is released at the time trad given in equation S12 at a

radius Rrad ≈ vtrad after adiabatic losses have reduced the internal energy to MvR0/trad and

60

the luminosity is L ∼MvR0/t2rad. Then using equation S12 for trad we find:

L ≈ 6× 1040 erg s−1κ−1 R0

1010 cm

(β

0.2

)2

(S10)

independent of the mass. We observed a luminosity of 1042 erg implying that in order to explain

the early emission as shock cooling, the shock should hit the ejecta at a radius of ∼ 1011 cm.

Since the ejecta propagates at a velocity that does not exceed∼ 0.2c it should be shocked∼ 20 s

after the merger. Such an option is plausible given that we know that in some sGRBs there is

engine activity that releases an amount of energy that is comparable to the one seen in the burst

over a duration of ∼ 100 s (31).

To verify our simple estimates we carried out a 1D numerical simulation with spherical

symmetry (using the same code as in section where we inject an engine-driven wind that lasts

for 20 s and contains a total energy of 2×1051 erg into 0.05M� of ejecta that expands initially

at a velocity of 0.1−0.2c. We found that this can produce the observed emission during the first

day for κ ≈ 3 cm2 g−1. We conclude that long-lasting engine-driven wind is another possible

explanation for the bright, blue emission of EM170817 seen during the first day.

Early-time emission powered by free neutron decay

The observed emission during the first day is brighter, bluer and rises faster than predicted. It

is not naturally explained by the standard models of radio-active decay of r-process elements

(see main text). It was suggested (124) that the decay of free neutrons may give rise to an early

blue emission. We therefore examine if decay of free neutrons may have made a significant

contribution to the UV/optical emission observed during the first day.

Consider an outflow with a mass m = 0.01M�m−2, a typical velocity v = 0.5cβ0.5 and an

initial free neutron fraction Xn. The evolution of internal energy confined to the flow, E, can

be estimated as:dE

dt=E

t+ Lh − Lrad (S11)

61

The first term takes into account adiabatic losses, the second heating term is due to neutron

decay, Lh = 6× 1045m−2Xne(−t/900 s) erg s−1, and the cooling term Lrad accounts for radiative

losses. The trapped radiation is released roughly at

trad ≈( κm

4πcv

)1/2

= 0.7m0.5−2κ

0.5β−0.50.5 day, (S12)

where opacity κ is in cm2 g−1. Up to t � trad equation S11 can be integrated neglecting

radiative losses. For 900 s � t � trad this integration yields E = 6 × 1046m−2Xntd−1 erg,

where td is time in days. The trapped radiation is radiated at trad with a peak luminosity that is

roughly Lpeak ≈ E/trad.

Lpeak ≈E(trad)

trad

≈ 2× 1042Xnκ−1β0.5 erg s−1 (S13)

Note that this luminosity is independent of m. The observed luminosity during the first day

implies that for κ = 1 cm2 g−1, Xn is 0.5 and the ejecta must be at least ∼ 0.01M� of almost

pure free neutrons. For larger κ neutrons cannot produce the observed signal while lower κ

reduces the neutron fraction, while increasing the total mass by the same factor (to match trad),

so an unrealistically high mass of ∼ 0.01M� of free neutrons is always needed to explain the

observations from the first day.

62

RatesEM170817-like events in the Palomar Transient Factory database

The Palomar Transient Factory (PTF; (125)) and intermediate Palomar Transient Factory (iPTF;

(126, 127)) databases keep track of every detection with significance greater than 5-σ on every

image subtraction performed during the 7 year surveys. This facilitates our ability to calculate

the rates of a variety of astrophysical events in hindsight. We determined the efficiency of ev-

ery image subtraction for a transient of a given magnitude sitting on top of a galaxy of a given

surface brightness for the PTF survey over the time frame of 2010 January through 2012 De-

cember (128). By coupling these efficiencies with a Monte Carlo simulation of a particular

transient event, we can calculate a rate based on comparison to the observed number of such

transients found during the survey or, if no such events were found, we can provide an upper

limit on this rate.

We used these efficiencies, coupled with the lightcurve for EM170817 to calculate the rate

in the following manner. First we performed a search in R-band for all transients resembling

EM170817. We only performed the search within the search radius of the CLU (Census of the

Local Universe; (7)) galaxy catalog within 200 Mpc as defined in (73). We required 3 detections

above our machine-learning threshold for a real source (129) found within a 10-day time frame

with at least one non-detection both the week prior to the event and the week after the event.

No other significant detections in the database were allowed at any time during the survey.

The result of this search yielded 2 potential candidate events, one of which is almost cer-

tainly a heretofore unreported nova in NGC 253. The other event, similar in absolute bright-

ness and decline rate as EM170817, will be the subject of a detailed rate study in future work

(Frohmaier et al., 2018 in preparation). For now we will assume there are no positive detections

of such events in PTF and proceed to calculate an upper limit for this rate.

From here we simulated, through a Monte Carlo, the lightcurves of EM170817 as seen in

63

our survey and if they would meet the required detection thresholds as defined above. The result

of this simulation is an event rate of 320 Gpc−3 yr−1 and a 3-σ upper limit of 800 Gpc−3 yr−1.

Relating this back to the optical counterparts of NS-NS mergers, we note that the major uncer-

tainty in all of this is the luminosity function in the R-band of such events. If the typical NS-NS

merger is 50% fainter, the upper limit on such events nearly doubles to over 1600 Gpc−3 yr−1.

A similar search was carried out in the iPTF database yielding no corresponding detections

using the techniques reported in (130). While the iPTF survey had a much higher cadence

than PTF, and should be better suited for the optical detection of rapidly-evolving events like

EM170817, we have yet to carry out a full study of the iPTF subtraction efficiencies, thus we

do not include this survey in our constraints on the rates. However, we do note that this would

make the rate reported above very conservative.

Implications for rates based on r-process abundances

Here we discuss the relation of GW/EM170817 to the origin of r-process elements in the Uni-

verse. The mass of the ejected r-process elements is estimated to be at least ∼ 0.05M� in

order to explain the observed light curves and spectra. The bright emission at early times sug-

gests that the Lanthanide-free material with a mass of > 0.02M� is ejected (with atomic mass

number A < 140; see figure S10). Given the ejecta mass and merger rate deduced above, we

can test the hypothesis that mergers produced all the r-process elements heavier than a mini-

mal atomic mass number, Amin > 70, in the Milky Way. The value of Amin for the merger

ejecta depends on the stiffness of the neutron star equation of state (131,132) and the lifetime of

the remnant massive neutron star ( (133), see also recent studies on the nucleosynthesis in the

merger ejecta (134, 135). Note that ∼ 80% of r-process elements are around the first r-process

peak (figure S10). Therefore the rate estimate from the Galactic r-process abundance is quite

sensitive to the choice of Amin. Assuming that the solar r-process abundance is the typical of

64

the Milky Way stars, we estimate the volumetric rate (136):

R ∼

500(

Mej

0.05M�

)−1

Gpc−3 yr−1 (Amin = 70),

100(

Mej

0.05M�

)−1

Gpc−3 yr−1 (Amin = 90),(S14)

where we use the number density of galaxies of ≈ 0.01 Mpc−3. The former corresponds to

that all the r-process elements are produced by mergers and the latter corresponds to that heavy

r-process elements without the first peak elements are produced. The large event rate and ejecta

mass inferred from GW/EM170817 suggest that all the r-process elements are predominantly

produced by mergers. Note that, however, the abundance pattern of the first peak elements of

extremely metal poor stars does not agree with each other (137). This suggests that more than

one type of astrophysical phenomenon may produce these elements or there is variation in the

composition and amount of the r-process ejecta of neutron star mergers.

65

Additional Acknowledgements

Gemini observatory data were obtained primarily under programs GS-2017B-DD-1 and GS-

2017B-DD-6 (PI L. P. Singer) and is available at https://archive.gemini.edu. CTIO1.3m data ob-

tained under program NOAO 2017B-0160 (PI B. Cobb) is available at http://archive.noao.edu/.

Hubble Space Telescope data obtained under program HST-GO-15436 (PI M. M. Kasliwal) is

available at https://archive.stsci.edu/. Keck Observatory data (PI G. Hallinan) is available at

https://koa.ipac.caltech.edu/. VLT data is available at http://archive.eso.org/.

The data of GALEX presented in this paper were obtained from the Mikulski Archive for

Space Telescopes (MAST). STScI is operated by the Association of Universities for Research

in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data

is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants

and contracts PS1: The Pan-STARRS1 Surveys (PS1) have been made possible through con-

tributions of the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project

Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for As-

tronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The

Johns Hopkins University, Durham University, the University of Edinburgh, Queen’s Univer-

sity Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory

Global Telescope Network Incorporated, the National Central University of Taiwan, the Space

Telescope Science Institute, the National Aeronautics and Space Administration under Grant

No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mis-

sion Directorate, the National Science Foundation under Grant No. AST-1238877, the Univer-

sity of Maryland, and Eotvos Lorand University (ELTE). 2MASS: This publication makes use

of data products from the Two Micron All Sky Survey, which is a joint project of the University

of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Tech-

66

nology, funded by the National Aeronautics and Space Administration and the National Science

Foundation. WISE: This publication makes use of data products from the Wide-field Infrared

Survey Explorer, which is a joint project of the University of California, Los Angeles, and the

Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronau-

tics and Space Administration. This research has made use of the NASA/IPAC Extragalactic

Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of

Technology, under contract with the National Aeronautics and Space Administration.

A.H. acknowledges support by the I-Core Program of the Planning and Budgeting Commit-

tee and the Israel Science Foundation. T.M. acknowledges the support of the Australian Re-

search Council through grant FT150100099. Parts of this research were conducted by the Aus-

tralian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO), through

project number CE110001020. The Australia Telescope Compact Array is part of the Australia

Telescope National Facility which is funded by the Australian Government for operation as

a National Facility managed by CSIRO. D.L.K. is additionally supported by NSF grant AST-

1412421. A.A.M. is funded by the Large Synoptic Survey Telescope Corporation in support

of the Data Science Fellowship Program. P.C.Y., C.C.N. and W.H.I. thank the support from

grant MOST104-2923-M-008-004-MY5 and MOST106-2112-M-008-007. A.C. acknowledges

support from the National Science Foundation CAREER award 1455090, “CAREER: Radio

and gravitational-wave emission from the largest explosions since the Big Bang”. T.P. acknowl-

edges the support of Advanced ERC grant TReX and the Templeton Foundation. B.E.C, thanks

SMARTS 1.3-m Queue Manager Bryndis Cruz for prompt scheduling of the SMARTS observa-

tions. Basic research in radio astronomy at the Naval Research Laboratory (NRL) is funded by

6.1 Base funding. Construction and installation of VLITE was supported by NRL Sustainment

Restoration and Maintenance funding. K.P.M’s research is supported by the Oxford Centre for

Astrophysical Surveys which is funded through the Hintze Family Charitable Foundation. J.S.

67

and A.G. are grateful for support from the Knut and Alice Wallenberg Foundation. Together

with S.R they are also supported by the research environment grant “Gravitational Radiation and

Electromagnetic Astrophysical Transients (GREAT)” funded by the Swedish Research Coun-

cil (VR) under Dnr 2016-06012. S.R. has also been supported by VR (grant number 2016-

036573), and by the Swedish National Space Board (107/16). E.O.O. is grateful for the support

by grants from the Israel Science Foundation, Minerva, Israeli ministry of Science, the US-

Israel Binational Science Foundation, and the I-CORE Program of the Planning and Budgeting

Committee and The Israel Science Foundation. We thank the staff of the GMRT that made

these observations possible. The GMRT is run by the National Centre for Radio Astrophysics

of the Tata Institute of Fundamental Research. A.Y.Q.H. was supported by a National Science

Foundation Graduate Research Fellowship under Grant No. DGE1144469. J.E.J. acknowledges

supported by the National Science Foundation Graduate Research Fellowship under grant no.

DGE-1144469. S.G. acknowledges support from NSF Award PHY-1607585. G.C.A. and V.B.

acknowledge the support of the Science and Engineering Research Board, Department of Sci-

ence and Technology, India, for the GROWTH-India project. J.B. acknowledges support from

the Einstein Fellowship grant PF7-180162. N.K., R.I. and Y.Y. acknowledge the support of

Japan Society for the Promotion of Science for the GROWTH project. CZTI is built by a TIFR-

led consortium of institutes across India, including VSSC, ISAC, IUCAA, SAC and PRL. The

Indian Space Research Organisation funded, managed and facilitated the project. Parts of this

research were conducted by the Australian Research Council Centre of Excellence for All-sky

Astrophysics in 3D (ASTRO 3D) through project number CE170100013. F.R. acknowledges

support from the U.S. Department of Energy Early Career Award DE-SC0012160 “Scalable

and Energy-Efficient Methods for Interactive Exploration of Scientific Data.” E.N. and O.G.

acknowledge the support of an ERC starting grant (GRB/SN) and an ISF grant (1277/13).

P.E.N. acknowledges support from the DOE through DE-FOA-0001088, “Analytical Modeling

68

for Extreme-Scale Computing Environments.” This research used resources of the National En-

ergy Research Scientific Computing Center, a DOE Office of Science User Facility supported

by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-

05CH11231.

69

Figure S1: Hubble Space Telescope WFC3/F336W ultraviolet image of EM170817 andNGC 4993, taken 2017 August 22. North is up, east is the to left, and a 5′′ scale-bar is in-dicated. The position of the transient is shown with tick marks. Dust lanes are visible towardthe center of NGC 4993.

70

Table S1: Archival and new ultraviolet, optical, and near-infrared photometry of EM170817.For each observation we give the observation date, time since GW170817, telescope, instru-ment, filter, and AB magnitude. Upper limits are all at 5-σ confidence.

Observation Date ∆t Telescope Instrument Filter AB Magnitude Reference(UTC) (d)

2017-08-17 23:31 0.45 Swope directCCD i 17.33± 0.10 (10, 146, 147)2017-08-18 00:01 0.47 PROMPT5 CCD R 17.52± 0.20 (148, 149)2017-08-18 00:04 0.47 CTIO DECam i 17.50± 0.10 (150, 151)2017-08-18 00:04 0.47 CTIO DECam z 17.50± 0.10 (150, 151)2017-08-18 00:10 0.48 ESO-VISTA VIRCAM J 18.42± 0.10 (152)2017-08-18 00:15 0.48 Magellan-Clay MEGACAM g 17.20± 0.10 (147, 153)2017-08-18 01:30 0.53 Gemini-S FLAMINGOS-2 Ks 18.42± 0.04 this paper2017-08-18 03:44 0.63 Swift UVOT UVM2 21.32± 0.22 (11)2017-08-18 03:54 0.63 Swift UVOT UVW1 19.62± 0.11 (11)2017-08-18 04:01 0.64 Swift UVOT U 18.23± 0.08 (11)2017-08-18 04:07 0.64 Swift UVOT UVW2 21.48± 0.25 (11)2017-08-18 05:33 0.70 Pan-STARRS GPC1 y 17.28± 0.13 (154, 155)2017-08-18 05:33 0.70 Pan-STARRS GPC1 z 17.31± 0.09 (154, 155)2017-08-18 05:34 0.70 Pan-STARRS GPC1 i 17.23± 0.10 (154, 155)2017-08-18 05:35 0.70 Pan-STARRS GPC1 y 17.47± 0.16 (154, 155)2017-08-18 05:36 0.71 Pan-STARRS GPC1 z 17.21± 0.08 (154, 155)2017-08-18 05:37 0.71 Pan-STARRS GPC1 i 17.26± 0.08 (154, 155)2017-08-18 05:40 0.71 Subaru HSC z 17.30± 0.10 (156)2017-08-18 09:42 0.88 SSO CCD g 17.50± 0.20 (157)2017-08-18 13:39 1.04 Swift UVOT UVW1 20.36± 0.21 (11)2017-08-18 13:43 1.04 Swift UVOT U 19.05± 0.16 (11)2017-08-18 13:48 1.05 Swift UVOT UVW2 > 21.19 (11)2017-08-18 14:11 1.06 AST3-2 CCD g 18.15± 0.10 (158, 159)2017-08-18 14:15 1.07 Swift UVOT UVM2 > 21.25 (11)2017-08-18 17:17 1.19 KMTNet 18KCCD V 17.83± 0.10 (160, 161)2017-08-18 17:17 1.19 KMTNet 18KCCD I 17.75± 0.10 (160, 161)2017-08-18 17:17 1.19 KMTNet 18KCCD R 17.62± 0.10 (160, 161)2017-08-18 17:17 1.19 MASTER-II NA R 17.42± 0.20 (162, 163)2017-08-18 17:34 1.20 MASTER-II NA B 18.42± 0.10 (162, 163)2017-08-18 20:42 1.33 NOT NOTCam J 17.13± 0.11 (164)2017-08-18 20:42 1.33 NOT NOTCam Ks 17.70± 0.25 (164)2017-08-18 23:45 1.46 KMTNet 18KCCD R 17.82± 0.10 (160, 161)2017-08-18 23:46 1.46 KMTNet 18KCCD I 17.85± 0.10 (160, 161)2017-08-18 23:46 1.46 KMTNet 18KCCD V 18.33± 0.10 (160, 161)2017-08-18 23:59 1.47 PROMPT5 CCD R 18.22± 0.06 (149, 165)2017-08-19 00:44 1.50 Swift UVOT U > 19.50 (11)2017-08-19 00:50 1.51 Swift UVOT UVW2 > 21.53 (11)2017-08-19 01:15 1.52 Swift UVOT UVM2 > 21.71 (11)71

Table S1: Archival and new ultraviolet, optical, and near-infrared photometry of EM170817(continued).

Observation Date ∆t Telescope Instrument Filter AB Magnitude Reference(UTC) (d)

2017-08-19 01:23 1.53 Swift UVOT UVW1 > 20.86 (11)2017-08-19 01:55 1.55 Swope directCCD B 18.70± 0.10 (10, 147, 166)2017-08-19 01:55 1.55 Swope directCCD V 18.14± 0.10 (10, 147, 166)2017-08-19 01:55 1.55 Swope directCCD i 17.43± 0.10 (10, 147, 166)2017-08-19 05:39 1.71 Subaru HSC z 17.80± 0.10 (167)2017-08-19 05:46 1.71 Pan-STARRS GPC1 i 17.91± 0.05 (155, 168)2017-08-19 05:46 1.71 Pan-STARRS GPC1 y 17.59± 0.07 (155, 168)2017-08-19 05:46 1.71 Pan-STARRS GPC1 z 17.80± 0.05 (155, 168)2017-08-19 10:59 1.93 Zadko AndorIKON-L r 18.46± 0.17 (159, 169)2017-08-19 14:09 2.06 Swift UVOT U > 21.02 (11)2017-08-19 14:17 2.07 Swift UVOT UVW2 > 21.55 (11)2017-08-19 14:25 2.07 Swift UVOT UVM2 > 21.59 (11)2017-08-19 14:33 2.08 Swift UVOT UVW1 > 21.57 (11)2017-08-19 17:17 2.19 KMTNet 18KCCD R 18.52± 0.10 (160, 161)2017-08-19 17:17 2.19 KMTNet 18KCCD V 19.43± 0.10 (160, 161)2017-08-19 17:17 2.19 KMTNet 18KCCD I 18.35± 0.10 (160, 161)2017-08-19 17:53 2.22 MASTER-II NA R 18.42± 0.30 (162, 163)2017-08-19 23:23 2.45 CTIO DECam i 17.80± 0.10 (151, 170, 171)2017-08-19 23:23 2.45 CTIO DECam z 17.60± 0.10 (151, 170, 171)2017-08-19 23:31 2.45 KMTNet 18KCCD R 18.62± 0.10 (160, 161)2017-08-19 23:31 2.45 KMTNet 18KCCD I 18.65± 0.10 (160, 161)2017-08-19 23:31 2.45 KMTNet 18KCCD V 19.83± 0.10 (160, 161)2017-08-19 23:50 2.46 PROMPT5 CCD r 18.96± 0.10 (149, 172)2017-08-20 00:19 2.49 Gemini-S FLAMINGOS-2 Ks 17.60± 0.04 this paper2017-08-20 00:27 2.49 Gemini-S FLAMINGOS-2 J 17.76± 0.02 this paper2017-08-20 05:46 2.71 Pan-STARRS GPC1 y 18.08± 0.07 (155, 173)2017-08-20 05:46 2.71 Pan-STARRS GPC1 i 18.47± 0.08 (155, 173)2017-08-20 05:46 2.71 Pan-STARRS GPC1 z 18.31± 0.06 (155, 173)2017-08-20 11:14 2.94 Zadko AndorIKON-L r 19.18± 0.12 (159, 169)2017-08-20 11:47 2.96 Swift UVOT UVW1 > 22.18 (11)2017-08-20 13:08 3.02 Swift UVOT U > 21.87 (11)2017-08-20 13:11 3.02 Swift UVOT B > 19.82 (11)2017-08-20 13:17 3.03 Swift UVOT UVW2 > 22.76 (11)2017-08-20 13:25 3.03 Swift UVOT V > 19.04 (11)2017-08-20 15:11 3.10 Swift UVOT UVM2 > 23.29 (11)2017-08-21 00:35 3.50 Gemini-S FLAMINGOS-2 J 17.93± 0.06 this paper2017-08-21 00:38 3.50 Gemini-S FLAMINGOS-2 H 17.72± 0.04 this paper2017-08-21 00:40 3.50 Gemini-S FLAMINGOS-2 Ks 17.61± 0.06 this paper

72

Table S1: Archival and new ultraviolet, optical, and near-infrared photometry of EM170817(continued).

Observation Date ∆t Telescope Instrument Filter AB Magnitude Reference(UTC) (d)

2017-08-21 05:31 3.70 Pan-STARRS GPC1 y > 18.00 (155, 174)2017-08-21 05:46 3.71 Pan-STARRS GPC1 z 18.27± 0.33 (155, 174)2017-08-21 05:46 3.71 Pan-STARRS GPC1 i > 18.50 (155, 174)2017-08-21 09:30 3.87 Zadko AndorIKON-L r 19.86± 0.21 (159, 169)2017-08-22 01:00 4.51 Gemini-S FLAMINGOS-2 Ks 17.72± 0.09 this paper2017-08-22 01:03 4.52 Gemini-S FLAMINGOS-2 J 18.15± 0.06 this paper2017-08-22 01:06 4.52 Gemini-S FLAMINGOS-2 H 18.02± 0.07 this paper2017-08-22 09:43 4.88 Swift UVOT UVW1 > 22.14 (11)2017-08-22 11:43 4.96 Zadko AndorIKON-L r 20.20± 0.23 (159, 169)2017-08-22 20:19 5.32 HST WFC3/UVIS F336W 24.97± 0.11 this paper2017-08-22 20:28 5.32 HST WFC3/UVIS F336W 25.05± 0.11 this paper2017-08-22 21:01 5.35 HST WFC3/UVIS F336W 25.18± 0.11 this paper2017-08-22 23:23 5.45 ESO-VISTA OMEGACAM i 21.31± 0.10 (175)2017-08-22 23:23 5.45 ESO-VISTA OMEGACAM r 22.43± 0.10 (175)2017-08-22 23:23 5.45 ESO-VISTA OMEGACAM g 23.27± 0.10 (175)2017-08-23 08:04 5.81 Swift UVOT U > 20.74 (11)2017-08-23 08:05 5.81 Swift UVOT B > 19.94 (11)2017-08-23 08:07 5.81 Swift UVOT UVW2 > 23.03 (11)2017-08-23 08:09 5.81 Swift UVOT V > 18.91 (11)2017-08-23 08:11 5.81 Swift UVOT UVM2 > 22.66 (11)2017-08-23 11:48 5.96 Zadko AndorIKON-L r > 20.60 (159, 169)2017-08-23 17:22 6.20 IRSF SIRIUS H 18.60± 0.18 this paper2017-08-23 17:22 6.20 IRSF SIRIUS Ks 18.01± 0.10 this paper2017-08-23 17:22 6.20 IRSF SIRIUS J 18.65± 0.19 this paper2017-08-23 23:35 6.45 VLT VISIR J8.9 > 8.26 this paper2017-08-24 11:55 6.97 Zadko AndorIKON-L r > 20.60 (159, 169)2017-08-24 16:51 7.17 IRSF SIRIUS J 18.95± 0.32 this paper2017-08-24 16:51 7.17 IRSF SIRIUS H 18.53± 0.17 this paper2017-08-24 16:51 7.17 IRSF SIRIUS Ks 18.02± 0.12 this paper2017-08-24 23:20 7.44 CTIO1.3m ANDICAM K 18.06± 0.17 this paper2017-08-25 11:52 7.97 Zadko AndorIKON-L r > 20.60 (159, 169)2017-08-25 16:42 8.17 Swift UVOT UVW1 > 22.51 (11)2017-08-25 23:29 8.45 CTIO1.3m ANDICAM K 18.44± 0.18 this paper2017-08-26 16:57 9.18 IRSF SIRIUS H 18.83± 0.23 this paper2017-08-26 16:57 9.18 IRSF SIRIUS Ks 18.25± 0.21 this paper2017-08-26 16:57 9.18 IRSF SIRIUS J > 18.87 this paper2017-08-26 22:56 9.43 Gemini-S FLAMINGOS-2 Ks 18.50± 0.08 this paper2017-08-26 23:01 9.43 Gemini-S FLAMINGOS-2 J 20.57± 0.20 this paper

73

Table S1: Archival and new ultraviolet, optical, and near-infrared photometry of EM170817(continued).

Observation Date ∆t Telescope Instrument Filter AB Magnitude Reference(UTC) (d)

2017-08-26 23:05 9.43 Gemini-S FLAMINGOS-2 H 19.68± 0.08 this paper2017-08-26 23:21 9.44 CTIO1.3m ANDICAM K 18.43± 0.17 this paper2017-08-27 02:15 9.57 APO NICFPS Ks > 17.99 this paper2017-08-27 02:49 9.59 Palomar5m WHIRC Ks > 17.64 this paper2017-08-27 23:07 10.43 Gemini-S FLAMINGOS-2 Ks 18.77± 0.07 this paper2017-08-27 23:10 10.44 Gemini-S FLAMINGOS-2 H 19.63± 0.08 this paper2017-08-27 23:16 10.44 Gemini-S FLAMINGOS-2 J 21.33± 0.30 this paper2017-08-27 23:18 10.44 CTIO1.3m ANDICAM K 18.91± 0.19 this paper2017-08-28 16:40 11.17 Swift UVOT B > 20.48 (11)2017-08-28 16:44 11.17 Swift UVOT UVW2 > 24.32 (11)2017-08-28 16:47 11.17 Swift UVOT V > 20.07 (11)2017-08-28 16:50 11.17 Swift UVOT UVM2 > 23.29 (11)2017-08-28 16:52 11.17 IRSF SIRIUS J > 18.37 this paper2017-08-28 16:52 11.17 IRSF SIRIUS Ks > 18.48 this paper2017-08-28 16:52 11.17 IRSF SIRIUS H > 18.43 this paper2017-08-28 17:21 11.20 Swift UVOT U > 21.44 (11)2017-08-28 23:17 11.44 CTIO1.3m ANDICAM K > 19.11 this paper2017-08-28 23:35 11.45 Gemini-S FLAMINGOS-2 Ks 19.41± 0.09 this paper2017-08-28 23:40 11.46 Gemini-S FLAMINGOS-2 H > 20.63 this paper2017-08-28 23:45 11.46 Gemini-S FLAMINGOS-2 J > 21.07 this paper2017-08-29 00:36 11.50 HST WFC3/UVIS F225W > 26.04 this paper2017-08-29 00:36 11.50 HST WFC3/UVIS F336W > 26.37 this paper2017-08-29 00:36 11.50 HST WFC3/UVIS F275W > 26.13 this paper2017-08-29 10:44 11.92 Swift UVOT UVW1 > 22.26 (11)2017-08-29 23:10 12.44 Gemini-S FLAMINGOS-2 Ks 19.44± 0.08 this paper2017-08-29 23:23 12.45 Gemini-S FLAMINGOS-2 H 20.99± 0.21 this paper2017-08-29 23:41 12.46 Gemini-S FLAMINGOS-2 J > 21.55 this paper2017-08-30 23:01 13.43 Gemini-S FLAMINGOS-2 Ks 19.84± 0.09 this paper2017-08-30 23:29 13.45 Gemini-S FLAMINGOS-2 H 21.48± 0.30 this paper2017-08-30 23:43 13.46 Gemini-S FLAMINGOS-2 J > 21.94 this paper2017-08-31 23:03 14.43 Gemini-S FLAMINGOS-2 Ks 20.06± 0.10 this paper2017-08-31 23:18 14.44 VLT VISIR J8.9 > 7.74 this paper2017-08-31 23:50 14.47 Gemini-S FLAMINGOS-2 H 21.63± 0.36 this paper2017-09-01 00:18 14.48 Gemini-S GMOS i > 23.20 this paper2017-09-01 23:18 15.44 VLT VISIR J8.9 > 7.57 this paper2017-09-01 23:24 15.45 Gemini-S FLAMINGOS-2 Ks 20.43± 0.13 this paper2017-09-02 00:46 15.50 Gemini-S GMOS i > 23.40 this paper2017-09-02 23:22 16.45 Gemini-S FLAMINGOS-2 Ks 20.95± 0.18 this paper

74

6000 7000 8000 9000 10000Wavelength (Å)

−0.5

0.0

0.5

1.0

1.5

2.0

2.5

Flux

(10−

17 e

rg c

m−2

s−1

Å−1

)

Figure S2: Optical spectrum of EM170817 taken 2017 August 25 with the Low ResolutionImaging Spectrometer on Keck I. The measured spectrum is shown in gray, and a versionsmoothed with a kernel of width 20 A is shown in red. EM170817 was already quite faintat this time and no unambiguous features are evident.

Table S1: Archival and new ultraviolet, optical, and near-infrared photometry of EM170817(continued).

Observation Date ∆t Telescope Instrument Filter AB Magnitude Reference(UTC) (d)

2017-09-03 00:03 16.47 Gemini-S GMOS r > 21.18 this paper2017-09-03 23:36 17.46 Gemini-S FLAMINGOS-2 Ks > 19.92 this paper2017-09-04 00:16 17.48 Gemini-S GMOS r > 21.98 this paper2017-09-04 23:28 18.45 Gemini-S FLAMINGOS-2 Ks 21.04± 0.09 this paper2017-09-05 00:03 18.47 Gemini-S GMOS i > 21.90 this paper2017-09-05 23:48 19.46 Gemini-S FLAMINGOS-2 Ks 21.23± 0.37 this paper2017-09-06 23:30 20.45 Gemini-S FLAMINGOS-2 H > 21.22 this paper2017-09-06 23:33 20.45 VLT VISIR J8.9 > 7.42 this paper2017-09-07 23:39 21.46 Gemini-S FLAMINGOS-2 Ks > 21.48 this paper2017-09-11 23:39 25.46 Gemini-S FLAMINGOS-2 J > 20.21 this paper2017-09-14 23:14 28.44 Gemini-S FLAMINGOS-2 Ks > 19.96 this paper2017-09-15 23:19 29.44 Gemini-S FLAMINGOS-2 Ks > 20.60 this paper

75

4000 5000 6000 7000 8000 9000 10000 11000Rest wavelength (Å)

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

Flux

den

sity,

f (1

016

erg

s1 c

m2 Å

1 )

Gemini-S/GMOS2.5days

Gemini-S/GMOS3.5days

Keck/LRIS7.71days

Figure S3: Optical spectral sequence of EM170817 including the Gemini-S/GMOS spectrafrom 2.5 and 3.5 days and the Keck/LRIS spectrum from 7.7 days. The measured spectra areshown in gray, and versions smoothed with a Savitsky-Golay filter are shown in black.

Table S2: Log of optical and near-IR spectroscopic observations of EM170817. For each ob-servation we give the observation date, the time since GW170817, the telescope, instrument,exposure time, approximate wavelength range, and spectral resolution.

Observation Date ∆t Telescope Instrument Exposure Wavelength Range λ/∆λ(UTC) (days) (s) (A)

2017-08-20 01:08 2.52 Gemini-S GMOS 2× 300 6000–9000 19002017-08-21 00:15 3.48 Gemini-S GMOS 4× 360 3800–9200 17002017-08-22 00:21 4.49 Gemini-S FLAMINGOS-2 8× 150 12980–25070 6002017-08-22 00:47 4.50 Gemini-S FLAMINGOS-2 6× 150 9840–18020 6002017-08-25 05:45 7.71 Keck I LRIS 300 + 2× 600 2000–10300 10002017-08-29 00:23 11.49 Gemini-S FLAMINGOS-2 16× 150 12980–25070 600

76

10000 12000 14000 16000 18000 20000 22000 24000Rest wavelength (Å)

0

1

2

3

4

5

Norm

alize

d flu

x + co

nsta

nt

SN2014J16 days

SN2014J94 days

SN2005df198 days

[Co

II][S

II]

[Ni II

][F

e III

][F

e II]

[Ni II

][F

e II]

[Co

III]

[Fe

II]

[Fe

II][C

o II]

[Co

III]

[Fe

II] [Co

III]

Figure S4: Near-infrared spectrum of EM170817 at t = 4.5 days post merger is shown at thebottom of the figure, along with the spectra of the type Ia SN 2014J at 16 and 94 days postmaximum (138), and the type Ia SN 2005df at 198 days post maximum (139). Each spectrumis normalized to the flux between 10000–10500 A and shifted up from the one below for clarity.The spectrum of EM170817 was corrected for Milky Way reddening assumingE(B−V ) = 0.1and a standard RV = 3.1 extinction law (140), and smoothed using a Savitzky–Golay filter toclearly show the prominent, broad spectral features at∼ 10600 and 15000 A. The unfiltered dataare shown in gray. Regions of low S/N due to the strong telluric absorption features betweenthe J , H , and K spectral windows are indicated by the vertical, gray bars. We label severaltransitions of Fe-peak elements on the top spectrum of SN 2014J. While qualitatively similar,the broad Fe-peak features characteristic of SNe Ia are inconsistent with the features observedin the spectrum of EM170817.

77

10000 12000 14000 16000 18000 20000 22000 24000Rest wavelength (Å)

0

1

2

3

4

5

Norm

alize

d flu

x + co

nsta

nt

SN1998bw(Ic-BL)8 days

SN2010jl(IIn)21 days

SN2011dh(IIb)

13 days

SN2013ej(IIP)12 days

Br He IHe I

++ Pa

Pa

O I

+ Na I

Figure S5: Near-infrared spectrum of EM170817 at t = 4.5 days post merger is shown at thebottom of the figure, along with the spectra of the type IIP SN 2013ej at 12 days (141), the typeIIb SN 2011dh at 13 days (142), the type IIn SN 2010jl at 21 days (143), and the broad-linedtype Ic (Ic-BL) SN 1998bw at 8 days post maximum (144). Each spectrum is normalized to theflux between 10800.0–10950 A and shifted up from the one below for clarity. The spectrum ofEM170817 was corrected for Milky Way reddening assuming E(B − V ) = 0.1 and a standardRV = 3.1 extinction law (140), and smoothed using a Savitzky–Golay filter to clearly showthe prominent, broad spectral features at ∼ 10600 and 15000 A. The unfiltered data are shownin gray. Regions of low S/N due to the strong telluric absorption features between the J , H ,and K spectral windows are indicated by the vertical, gray bars. We label several transitionscommonly identified in the spectra of core-collapse SNe with vertical, dashed lines.

78

0 2 4 6 8 10 12 14 16 18 20Time since GW170817 (days)

14

16

18

20

22

24

26

28

30

32

Appa

rent

mag

nitud

e (A

B)

UVW1+10UVM2,F225W+7UVW2+6U,F336W,u+7B,g0+6g+5V+4

r0,r,R+3i,i 0,I+2z,z0+1y−0J−1H−2K,Ks−3

InterpolatedMCMC fitMCMC extrapolated −17.5

−15.0

−12.5

−10.0

−7.5

−5.0

−2.5

0.0

Abso

lute

mag

nitud

e (A

B)

Figure S6: Multiwavelength lightcurve based on the ultraviolet/optical/infrared photometry ofEM170817. The data are our assembled photometry (Table S1) plotted as AB magnitude vs.time since GW170817, with open triangles indicating 5σ upper limits, colored by wavelength.We plot both apparent magnitude and absolute magnitudes assuming distance of 40 Mpc. Weillustrate both methods of blackbody fitting used here: the solid curves are the low-order inter-polants used at each wavelength to fit a separate absorbed blackbody at every epoch, while thedashed curves are the evolving absorbed blackbodies fit simultaneously with a Markov-ChainMonte Carlo (MCMC) algorithm (the dotted portions are extrapolations). Filters with similarwavelengths (roughly within 300 A) have been grouped together for clarity, and some filtershave been omitted.

79

Figure S7: Spectral energy distribution obtained by combining photometric data of GALEX,Pan-STARRS, 2MASS, WISE, and IRAS surveys (red squares). The flux density was correctedfor Galactic extinction before the fitting assuming RV = 3.1 and E(B − V ) = 0.1. We usedupper limits for GALEX/FUV and IRAS/60µm bands. The upper panel shows the unattenuatedstellar spectrum (blue line) and the sum of attenuated stellar spectrum and the infrared emission(black line). The lower panel shows the residuals (Lobs

λ − Lmodelλ )/Lobs

λ (black squares), whereLobsλ and Lmodel

λ represent observed and predicted broad-band luminosity densities respectively.

80

103

104

105

106

E/E‘

0.05

0.06

0.07

0.08

0.09

0.1

0.11

0.12

ob

s-

j [=

min

]

obs-

j <

j

j<1/

Figure S8: The maximal value of θobs − θj as a function of the amplification parameter A ≡E/E ′ for observers far and close to the jet axes (cases (i) and (iii) in equation S2). The observedvalues of EM170817 that we used are photon energy E ′p = 185 keV and γ-ray spectral indexα = −0.6 (3, 4).

81

103

104

105

106

E/E‘

101

102

103

min

obs-

j <

j

Figure S9: The minimal Lorentz factor of the jet, Γmin, required for the jet to be optically thinas a function of the amplification parameter A ≡ E/E ′ for far off-axis observers, θobs−θj � θj

(case (i) in equation S2). The parameters assumed here were the same as those assumed infigure S8: photon energy E ′p = 185 keV and γ-ray spectral index α = −0.6 (3, 4).

82

Tabl

eS3

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83

Tabl

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Uni

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pc)

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PSE

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sitio

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84

10-11

10-10

10-9

10-8

10-7

10-6

80 100 120 140 160 180 200 220

1st p

eak

2nd

peak

Lant

hani

de

3rd

peak

Sola

r r-p

roce

ss a

bund

ance

Atomic mass number

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

80 100 120 140 160 180 200 220

1st p

eak

2nd

peak

Lant

hani

de

3rd

peak

Cum

ulat

ive fr

actio

n

Atomic mass number

Figure S10: Top: the solar abundance pattern of r-process elements (145). Bottom: their cumu-lative distribution.

85